Restore from omnibus-gitlab made backup

In https://gitlab.com/charts/helm.gitlab.io/issues/28 we have a backup/restore meta issue.

With #340 (closed) we are talking about restoring from backup.

However, these 2 issues are assuming existing installation using charts in this repository.

I suggest we look into what is specifically required to move from a VM based omnibus-gitlab installation into chart based installation.

This means that we need to look into a way of getting the backup archive extracted and placed in the correct locations to have a working setup.