Reference config properties specific to the system (#38)

With this commit we introduce new configuration properties that allow
Rally to change behavior (e.g. which build command is executed) based on
template variables. This enables us to build and download
system-specific artifacts which is needed e.g. in order to use the
bundled JDK.

cars/v1/vanilla/config.ini

@@ -1,5 +1,8 @@
 [variables]
+# deprecated
 release_url = https://download.elastic.co/elasticsearch/elasticsearch/elasticsearch-{{VERSION}}.tar.gz
+# new
+jdk.unbundled.release_url = https://download.elastic.co/elasticsearch/elasticsearch/elasticsearch-{{VERSION}}.tar.gz
 # list of JDK major versions that are used to run Elasticsearch
 runtime.jdk = 8,7
 runtime.jdk.bundled = false