Implement key rotation tests for OSD units

This PR adds OSD tests to the key rotation class for the ceph-mon
charm. It will be tested in the following merge requests:
https://review.opendev.org/c/openstack/charm-ceph-mon/+/915925
https://review.opendev.org/c/openstack/charm-ceph-osd/+/915926

zaza/openstack/charm_tests/ceph/tests.py

@@ -1802,6 +1802,7 @@ class CephMonKeyRotationTests(test_utils.BaseCharmTest):
         """Test that rotating the keys actually changes them."""
         unit = 'ceph-mon/0'
         self._check_key_rotation('mgr', unit)
+        self._check_key_rotation('osd.0', unit)
 
         try:
             zaza_model.get_application('ceph-radosgw')