Generate configuration from mage for all Beats (elastic#12618)

From now on all Beats use `mage` when generating configuration files. This comes with the following changes:
* Heartbeat does not use `post_process_config.py` to modify processors, instead it is added using the templating
* Functionbeat does not include unsupported outputs
* All Beats can alter the common configuration coming from `libbeat`.

### What does it mean for Community Beats?
When adopting this version of `libbeat`, you need to make sure that you have a target named `Config` in your `magefile.go`. An example target which generates a sample configuration, a reference configuration and a Docker configuration from `{yourbeat}/_meta/beat.yml` and the common configuration from `libbeat` is the following:
```golang
import devtools "github.com/elastic/beats/dev-tools/mage"

func Config() error {
    return devtools.Config(devtools.AllConfigTypes, devtools.ConfigFileParams{}, ".")
}
```
If the configuration of your Beat consists of multiple files, you can pass your own instance of `ConfigFileParams`:
```golang
func ConfigFileParams() devtools.ConfigFileParams {
    return devtools.ConfigFileParams{
        ShortParts: []string{
            devtools.OSSBeatDir("_meta/beat.yml"),
            devtools.OSSBeatDir("_meta/beat.custom.yml"),
            devtools.LibbeatDir("_meta/config.yml.tmpl"),
        },
        ReferenceParts: []string{
            devtools.OSSBeatDir("_meta/beat.reference.yml"),
            devtools.OSSBeatDir("_meta/beat.custom.referenceyml"),
            devtools.LibbeatDir("_meta/config.reference.yml.tmpl"),
        },
        DockerParts: []string{
            devtools.OSSBeatDir("_meta/beat.docker.yml"),
            devtools.LibbeatDir("_meta/config.docker.yml"),
        },
    }
}
```
It is also possible to pass variables to the templates using the `ExtraVars` attribute of `ConfigFileParams` struct.

_meta/beat.reference.yml

@@ -0,0 +1,52 @@
+###################### Journalbeat Configuration Example #########################
+
+# This file is an example configuration file highlighting only the most common
+# options. The journalbeat.reference.yml file from the same directory contains all the
+# supported options with more comments. You can use it as a reference.
+#
+# You can find the full configuration reference here:
+# https://www.elastic.co/guide/en/beats/journalbeat/index.html
+
+# For more available modules and options, please see the journalbeat.reference.yml sample
+# configuration file.
+
+#=========================== Journalbeat inputs =============================
+
+journalbeat.inputs:
+  # Paths that should be crawled and fetched. Possible values files and directories.
+  # When setting a directory, all journals under it are merged.
+  # When empty starts to read from local journal.
+- paths: []
+
+  # The number of seconds to wait before trying to read again from journals.
+  #backoff: 1s
+  # The maximum number of seconds to wait before attempting to read again from journals.
+  #max_backoff: 20s
+
+  # Position to start reading from journal. Valid values: head, tail, cursor
+  seek: cursor
+  # Fallback position if no cursor data is available.
+  #cursor_seek_fallback: head
+
+  # Exact matching for field values of events.
+  # Matching for nginx entries: "systemd.unit=nginx"
+  #include_matches: []
+
+  # Optional fields that you can specify to add additional information to the
+  # output. Fields can be scalar values, arrays, dictionaries, or any nested
+  # combination of these.
+  #fields:
+  #  env: staging
+
+
+#========================= Journalbeat global options ============================
+#journalbeat:
+  # Name of the registry file. If a relative path is used, it is considered relative to the
+  # data path.
+  #registry_file: registry
+
+#==================== Elasticsearch template setting ==========================
+setup.template.settings:
+  index.number_of_shards: 1
+  #index.codec: best_compression
+  #_source.enabled: false

journalbeat.reference.yml

@@ -308,7 +308,7 @@ setup.template.settings:
 
 #============================= Elastic Cloud ==================================
 
-# These settings simplify using journalbeat with the Elastic Cloud (https://cloud.elastic.co/).
+# These settings simplify using Journalbeat with the Elastic Cloud (https://cloud.elastic.co/).
 
 # The cloud.id setting overwrites the `output.elasticsearch.hosts` and
 # `setup.kibana.host` options.
@@ -434,7 +434,6 @@ output.elasticsearch:
   # never, once, and freely. Default is never.
   #ssl.renegotiation: never
 
-
 #----------------------------- Logstash output ---------------------------------
 #output.logstash:
   # Boolean flag to enable or disable the output module.
@@ -578,7 +577,7 @@ output.elasticsearch:
   #username: ''
   #password: ''
 
-  # Kafka version journalbeat is assumed to run against. Defaults to the "1.0.0".
+  # Kafka version Journalbeat is assumed to run against. Defaults to the "1.0.0".
   #version: '1.0.0'
 
   # Configure JSON encoding
@@ -835,7 +834,7 @@ output.elasticsearch:
   #filename: journalbeat
 
   # Maximum size in kilobytes of each file. When this size is reached, and on
-  # every journalbeat restart, the files are rotated. The default value is 10240
+  # every Journalbeat restart, the files are rotated. The default value is 10240
   # kB.
   #rotate_every_kb: 10000
 
@@ -847,7 +846,6 @@ output.elasticsearch:
   # Permissions to use for file creation. The default is 0600.
   #permissions: 0600
 
-
 #----------------------------- Console output ---------------------------------
 #output.console:
   # Boolean flag to enable or disable the output module.
@@ -863,26 +861,26 @@ output.elasticsearch:
 
 #================================= Paths ======================================
 
-# The home path for the journalbeat installation. This is the default base path
+# The home path for the Journalbeat installation. This is the default base path
 # for all other path settings and for miscellaneous files that come with the
 # distribution (for example, the sample dashboards).
 # If not set by a CLI flag or in the configuration file, the default for the
 # home path is the location of the binary.
 #path.home:
 
-# The configuration path for the journalbeat installation. This is the default
+# The configuration path for the Journalbeat installation. This is the default
 # base path for configuration files, including the main YAML configuration file
 # and the Elasticsearch template file. If not set by a CLI flag or in the
 # configuration file, the default for the configuration path is the home path.
 #path.config: ${path.home}
 
-# The data path for the journalbeat installation. This is the default base path
-# for all the files in which journalbeat needs to store its data. If not set by a
+# The data path for the Journalbeat installation. This is the default base path
+# for all the files in which Journalbeat needs to store its data. If not set by a
 # CLI flag or in the configuration file, the default for the data path is a data
 # subdirectory inside the home path.
 #path.data: ${path.home}/data
 
-# The logs path for a journalbeat installation. This is the default location for
+# The logs path for a Journalbeat installation. This is the default location for
 # the Beat's log files. If not set by a CLI flag or in the configuration file,
 # the default for the logs path is a logs subdirectory inside the home path.
 #path.logs: ${path.home}/logs
@@ -1087,7 +1085,7 @@ setup.kibana:
 # Send all logging output to Windows Event Logs. The default is false.
 #logging.to_eventlog: false
 
-# If enabled, journalbeat periodically logs its internal metrics that have changed
+# If enabled, Journalbeat periodically logs its internal metrics that have changed
 # in the last period. For each metric that changed, the delta from the value at
 # the beginning of the period is logged. Also, the total values for
 # all non-zero internal metrics are logged on shutdown. The default is true.
@@ -1134,7 +1132,7 @@ logging.files:
 
 
 #============================== Xpack Monitoring ===============================
-# journalbeat can export internal metrics to a central Elasticsearch monitoring
+# Journalbeat can export internal metrics to a central Elasticsearch monitoring
 # cluster.  This requires xpack monitoring to be enabled in Elasticsearch.  The
 # reporting is disabled by default.
 

journalbeat.yml

@@ -98,7 +98,7 @@ setup.kibana:
 
 #============================= Elastic Cloud ==================================
 
-# These settings simplify using journalbeat with the Elastic Cloud (https://cloud.elastic.co/).
+# These settings simplify using Journalbeat with the Elastic Cloud (https://cloud.elastic.co/).
 
 # The cloud.id setting overwrites the `output.elasticsearch.hosts` and
 # `setup.kibana.host` options.

magefile.go

@@ -226,3 +226,8 @@ func selectImage(platform string) (string, error) {
 
 	return devtools.BeatsCrossBuildImage + ":" + goVersion + "-" + tagSuffix, nil
 }
+
+// Config generates both the short/reference/docker configs.
+func Config() error {
+	return devtools.Config(devtools.AllConfigTypes, devtools.ConfigFileParams{}, ".")
+}