Fix running of migrations tests by Jenkins gate

DB schema migrations tests are always skipped for MySQL
and PostgreSQL backends, when running gate jobs. This is
due to the bug in the function, which checks, if given DB
backend is available.

Fixes bug 1205386

Change-Id: I80d5da64347279b8fb5a975ff0d11f31fe35c6ce
Merge "Fix running of migrations tests by Jenkins gate"