Add option to disable migrations completely
This would have helped with gitlab-com/operations#355 and is necessary for gitlab-com/operations#98 (closed).
Right now /etc/gitlab/skip-auto-migrations
only disables migrations for the post-install step, but a reconfigure will still attempt db:migrate if Rails detects a missing migration.
We need an omnibus option to turn off migrations completely. If we always had a blessed worker do the migrations, none of them would ever attempt to migrate the DB, which would be lead to less confusion and problems.