Fix default promtail manifest

[jbaumsap]

YAML structure is now as defined in spec, see
https://github.com/cloudfoundry-community/loki-boshrelease/blob/cfa473b8312077a963efd6f96a9672830beec0e6/jobs/promtail/spec

Without this I ran into an issue during "Rendering templates":

Task 387905 | 11:36:41 | Preparing deployment: Rendering templates (00:00:01)
                       L Error: Unable to render instance groups for deployment. Errors are:
  - Unable to render jobs for instance group 'loki'. Errors are:
    - Unable to render templates for job 'promtail'. Errors are:
      - Error filling in template 'config/config.yml.erb' (line 56: undefined method `each_pair' for an instance of Array)

Also line 56: Can't find property '["promtail.loki.external_labels"]'.

[jorbaum]

Closing in favor of #25 .