Make git account management optional

I'm currently working on deploying GitLab with NFS-backed storage in an environment using centralized account management. It would be very beneficial if the module had an option to allow management of the git account to be turned off.

In our case, we wish to use a service account defined in our directory so that the IDs match up between different hosts, including our filers.

Ideally, we need User[$git_user] to be optional, but it may be sufficient if the user's ID could be specified.