Adding new test case for zone test suite

1) test_serial_changes_on_update
   Test Scenario:
	# Update Zone's email - Serial has changed
        # Update Zone's TTL - Serial has changed
        # Update Zone's email and description - Serial has changed
        # Update Zone's description - Serial remained the same
   Note: The Description is a metadata field only known to Designate

Change-Id: I7461326d795221c1da4b7ac627d1ffec7c9212df
diff --git a/designate_tempest_plugin/common/constants.py b/designate_tempest_plugin/common/constants.py
index 4f6c72b..44e023f 100644
--- a/designate_tempest_plugin/common/constants.py
+++ b/designate_tempest_plugin/common/constants.py
@@ -12,14 +12,16 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-# Designate statuses strings
+# Designate statuses/actions strings
 PENDING = 'PENDING'
 COMPLETE = 'COMPLETE'
 ERROR = 'ERROR'
 DELETED = 'DELETED'
+DELETE = 'DELETE'
 ACTIVE = 'ACTIVE'
 UP = 'UP'
 CREATE = 'CREATE'
+UPDATE = 'UPDATE'
 
 # Zone types
 PRIMARY_ZONE_TYPE = 'PRIMARY'