allow specification of a custom template

[lobeck]

we haven't managed to pass relabel configs without weird encoding issues (. and other regex chars get interpreted and saved as values like \x5c.
so we've added a parameter to override the used template and hardcode the stuff in the template to avoid the weird hiera yaml decode and template yaml encode

example: prometheus::config_template: 'components/prometheus/aws.yaml.erb'

whereas the template might look like (this is actual example for some annoying apache storm metric rewriting)

<% require 'yaml' -%>
<% global_config = scope.lookupvar('::prometheus::global_config') -%>
<% rule_files = scope.lookupvar('::prometheus::rule_files') -%>
<% full_config = { 'global'=>global_config, 'rule_files'=>rule_files} -%>
<%= full_config.to_yaml -%>

    scrape_configs:
      - job_name: some-job-name
      scrape_interval: "5s"
      scrape_timeout: "5s"
      static_configs:
        - targets:
            - "foo:1234"
            - "bar:1234"
            - "baz:1234"
        - labels:
            alias: "Awesome Production"
      metric_relabel_configs:
        - source_labels: ['operation']
          regex:         '\"(.*)\..*\"'
          target_label:  'metric'
          replacement:   '$1'
        - source_labels: ['operation']
          regex:         '\".*\.(.*)\"'
          target_label:  'sub_metric'
          replacement:   '$1'
        - source_labels: ['operation']
          target_label:  'operation'
          replacement:   ''

[brutus333]

Looks good. I will merge this PR soon/