Bring back the cache secrets template

Bring back the gitlab-runner.cache.secret template that was removed in !393 (merged) in the last form it had there, just:

{{- if include "gitlab-runner.cache.secret" . }}
 - secret:
   name: {{ include "gitlab-runner.cache.secret" . }}
{{- end }}

This is in order for the gitlab helm chart to be able to optionally set the minio secret IF the default minio is enabled.

For end users, they can use the .secrets config to set these, but if the GitLab helm chart setup a default .secrets value for minio the install would start failing for anyone who had minio disabled. We need to be able to provide the default minio secret, but only when minio is enabled.