Move "opportunistic" db migrations tests from Nova

For db migrations tests on Jenkins we must use
hardcoded credentials and skip test if backend
(mysql or postgres) not available on developer
host.
Patch moves this "opportunistic" tests from Nova.

blueprint db-migration-tests-opportunistic

Change-Id: I3bd0be61a00916507f02a86a2e45c5831b3aaa5f
Merge "Move "opportunistic" db migrations tests from Nova"