Skip security_checklist test for xenial_mitaka

See bug 1915293 [0] for more details, but this test is already skipped
in the repo's current state, this is just making it more explicit as
part of the current refactor.

[0] https://bugs.launchpad.net/charm-openstack-dashboard/+bug/1915293

zaza/openstack/charm_tests/openstack_dashboard/tests.py

@@ -535,10 +535,15 @@ class SecurityTests(test_utils.OpenStackBaseTest,
         ]
         expected_passes = [
             'disable_password_autocomplete',
-            'enforce-password-check',
             'validate-file-ownership',
             'validate-file-permissions'
         ]
+        if (openstack_utils.get_os_release() <=
+                openstack_utils.get_os_release('xenial_mitaka')):
+            logging.info("Test expected to fail until bug #1915293 is fixed")
+            expected_failures.append('enforce-password-check')
+        else:
+            expected_passes.append('enforce-password-check')
 
         logging.info('Running `security-checklist` action'
                      ' on {} leader'.format(self.application_name))