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

cinder/tests/test_migrations.py

@@ -54,7 +54,7 @@ def _get_connect_string(backend,
     if backend == "postgres":
         backend = "postgresql+psycopg2"
 
-    return ("%(backend)s://%(user)s:%(passwd)s@localhost/%(database)s",
+    return ("%(backend)s://%(user)s:%(passwd)s@localhost/%(database)s" %
             {'backend': backend, 'user': user, 'passwd': passwd,
              'database': database})
 
@@ -79,6 +79,7 @@ def _is_backend_avail(backend,
     except Exception:
         # intentionally catch all to handle exceptions even if we don't
         # have any backend code loaded.
+        LOG.exception("Backend %s is not available", backend)
         return False
     else:
         connection.close()