Add omnibusConfigRuby value for gitlab-omnibus chart
This allows using additional omnibus configuration settings which are not directly supported by this helm chart (like smtp settings). This was implemented in the "gitlab" chart, but was missing in the gitlab-omnibus chart.
Example for values.yaml:
omnibusConfigRuby: |
gitlab_rails['smtp_enable'] = true
gitlab_rails['smtp_address'] = "smtp.example.org"
This should solve #65.