Add gitaly RunIt service to GitLab Omnibus

Closes: #1832 (closed)

The package does not include the executable itself at this time.

We intend to initially ship it seperatly, to allow it to iterate in production on a seperate schedule from GitLab, at least initially.

But if you have the executable installed, and you enable the service, It should now be run using RunIt and available during gitlab-ctl status/up/down/restart