Skip migrations test_walk_versions instead of pass

When there is no db engine configured for the migration tests,
the test_walk_versions test was passing, rather than skipping,
because it contained a for-loop and no check for the length of the
sequence being iterated over.

This patch checks the length of the configured engines and
properly skips the test when there are no engines.

Change-Id: I8a5c0a66f65a99c5169b251be02a8fbe6c727641
Closes-bug: #1327397
Merge "Skip migrations test_walk_versions instead of pass"