diskimage-retrofit use Focal not Bionic

If the test uses bionic source image the charm has not be able
to determine ubuntu_release which it cant always do. This
causes breakage when the image is bionic because it has to apply
ussuri uca for retrofit to work.

zaza/openstack/charm_tests/octavia/diskimage_retrofit/tests.py

@@ -55,7 +55,7 @@ def test_retrofit_image_source_image(self):
         glance = openstack.get_glance_session_client(session)
 
         for image in glance.images.list(filters={'os_distro': 'ubuntu',
-                                                 'os_version': '18.04'}):
+                                                 'os_version': '20.04'}):
             action = zaza.model.run_action(
                 'octavia-diskimage-retrofit/0',
                 'retrofit-image',