Add NotImplementedError to the abstract method

is_resource_deleted in the RestClient is abstract.
Abstract methods should throw NotImplementedError.

Change-Id: I13f98a6d99d90308d33dfa55da3f52532dd45706
diff --git a/tempest/common/rest_client.py b/tempest/common/rest_client.py
index 5710f4c..8fc5545 100644
--- a/tempest/common/rest_client.py
+++ b/tempest/common/rest_client.py
@@ -323,7 +323,9 @@
         """
         Subclasses override with specific deletion detection.
         """
-        return False
+        message = ('"%s" does not implement is_resource_deleted'
+                   % self.__class__.__name__)
+        raise NotImplementedError(message)
 
 
 class RestClientXML(RestClient):