Disable the pristine disk check on focal

The test won't pass on serverstack due to the snapd taking a copy of the
/mnt/vdb info at boot time and thus not being able to umount it is root
without also running through all of the procs and checking if they also
have it mounted.  Thus, this just disables the test on focal and newer.

zaza/openstack/charm_tests/ceph/tests.py

@@ -371,6 +371,13 @@ class CephTest(test_utils.OpenStackBaseTest):
         As the ephemeral device will have data on it we can use it to validate
         that these checks work as intended.
         """
+        current_release = zaza_openstack.get_os_release()
+        focal_ussuri = zaza_openstack.get_os_release('focal_ussuri')
+        if current_release >= focal_ussuri:
+            # NOTE(ajkavanagh) - focal (on ServerStack) is broken for /dev/vdb
+            # and so this test can't pass
+            logging.warn("Skipping pristine disk test for focal and higher")
+            return
         logging.info('Checking behaviour when non-pristine disks appear...')
         logging.info('Configuring ephemeral-unmount...')
         alternate_conf = {