Ignore Not Found when deleting Keystone role assignment

If the project has already been deleted, don't let that prevent role
assignments on it from being deleted.

Change-Id: I56aede8209e425ee6c2d762a44db8cda5416e69b
Task: 30955

heat/engine/resources/openstack/keystone/role_assignments.py

@@ -397,7 +397,8 @@ class KeystoneUserRoleAssignment(resource.Resource,
         self.update_assignment(user_id=self.user_id, prop_diff=prop_diff)
 
     def handle_delete(self):
-        self.delete_assignment(user_id=self.user_id)
+        with self.client_plugin().ignore_not_found:
+            self.delete_assignment(user_id=self.user_id)
 
     def validate(self):
         super(KeystoneUserRoleAssignment, self).validate()
@@ -455,7 +456,8 @@ class KeystoneGroupRoleAssignment(resource.Resource,
         self.update_assignment(group_id=self.group_id, prop_diff=prop_diff)
 
     def handle_delete(self):
-        self.delete_assignment(group_id=self.group_id)
+        with self.client_plugin().ignore_not_found:
+            self.delete_assignment(group_id=self.group_id)
 
     def validate(self):
         super(KeystoneGroupRoleAssignment, self).validate()