Add alerting when GitHost machines have downed migrations
When users upgrade to GitLab 9.0, the upgrade to PostgreSQL 9.6 may fail and cause Omnibus to revert back to 9.2, where migrations have not been run (https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2130). This will cause Error 500s for customers who attempt to use GitLab.
We should add alerting whenever a system has downed migrations for over some time (e.g. 30 minutes).