Replace GitLab Provisioner with GET in omnibus-gitlab pipelines

We currently use GitLab Provisioner in two places in omnibus-gitalb pipelines

1. As part of triggered package job. There is a manual job which users can play to test their MR in an HA mode (we spin up a PG HA cluster and run QA against it)
2. As part of nightly pipelines. We automatically spin up a PG HA cluster with nightly packages and run QA against it.

Now that we have consolidated all efforts to GET, we should phase out GitLab Provisioner in favor of it.