Additional Mongo configuration for SSL, additional metrics, and database connection

[bflad]

This PR copies additional available configuration found upstream here:
https://github.com/DataDog/dd-agent/blob/master/conf.d/mongo.yaml.example

Given this hiera and include datadog_agent::integrations::mongo:

datadog_agent::integrations::mongo::servers:
  - additional_metrics:
      - top
    database: my_db
    host: 127.0.0.1
    port: 27017
    ssl: true
    tags:
      - "replica_set:foo"

It generated this mongo configuration:

#
# MANAGED BY PUPPET
#

init_config:

instances:
  - server: mongodb://127.0.0.1:27017/my_db
    tags:
      - replica_set:foo
    ssl: true
    additional_metrics:
      - top

And reported as stable in the 5.6.3 agent after restart:

service datadog-agent info
===================
Collector (v 5.6.3)
===================
...
  Checks
  ======
...
    mongo
    -----
      - instance #0 [OK]
      - Collected 112 metrics, 0 events & 2 service checks
      - Dependencies:
          - pymongo: 2.8

Along with seeing metrics in the DataDog dashboard. Please let me know if you need anything else to get this merged! 🎉

[truthbk]

@bflad Awesome! Thanks a lot for your contribution! We'll be reviewing and testing this soon, and hopefully we can get this into the next release.

[truthbk]

Included in #176, thanks!