Fix mysql functional migration tests
Fix mysql functional migration tests

In patch [1] I marked all MySQL related migration tests
as unstable but I made mistake with calling super() method in
tests from TestModelsMigrationsMysql class.
This wasn't catched as tests were skipped instead of fail.

This patch fixes that, so tests can be run properly and be skipped
only if "real" issue with timeout will happen.

[1] https://review.openstack.org/#/c/624685/

Change-Id: Id42d6d8e24fdea5ac83f0fecc6975c19b933a501
Related-Bug: #1687027

openstack/neutron,mahak/neutron,noironetworks/neutron,noironetworks/neutron,openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron