Opportunistic migration tests

Migrations should be tested with real database backends. For this goal
intended number of base test cases which used in current implementation.
Previously there were two ways to run migration tests: using
opportunistic test cases (default way we've been using on CI) and
by using database connection strings, specified in test_migrations.conf,
for every particular database test case. For the sake of simplicity and
consistency we are moving to using of opportunistic db test cases here.

With this change we are free from locking, so we don't need `lockfile`
anymore.

Closes-Bug: #1327397
Closes-Bug: #1328997
Change-Id: I92b1dcd830c4755f429a0f6529911e607c2c7de7
Merge "Opportunistic migration tests"