In Kilo, we made a point to not allow any subtractive DB migrations. That
gave us a lot of flexibility for upgrades from Juno. Reviewers did the
awesome job of keeping to that plan. However, we need a mechanical filter
to prevent regressions. This adds that in the form of a banned-ops fixture
in test_migrations.
It excludes migrations from pre-Kilo times, as well as excludes a few
migrations that are already in place and do reaspnable things.
In general, we need to be extremely careful about letting migrations
put themselves on the exclusion list, so I added scary words around it.
Change-Id: Iefe678d0aeb1ad898fc5a0cee10fd55ccb7e06db
385284dad9