tests: Pin sahara-tests repo to ~Queens

The 0.6.0 tag on the repo seems to relate approximately
to the Queens release date, so we pin the repo there to
ensure that the tests continue to work ad infinitum.

Change-Id: I5da2ffcd4d4f330778707b2edd639089559ed778

tests/os_tempest-overrides.yml

@@ -20,7 +20,7 @@ tempest_run: yes
 tempest_plugins:
   - name: sahara-tests
     repo: https://git.openstack.org/openstack/sahara-tests
-    branch: "a2027280cd66f7dcc2437318e057158d1db5b625"
+    branch: "0.6.0"
 
 neutron_provider_networks:
   network_types: "vxlan,flat"