Ability to ignore db migrations

There are cases where migrations should not be executed. For example when
upgrading a zone, and the identity database is replicated across many zones.
Having the ability to toggle migration execution is useful (and a use-case
we have :))

Change-Id: I9d820e853d8afa13a55dbd49e5335d34246d97ba
Merge "Ability to ignore db migrations"