Fix stack update for Python API

Change I30a2b223d7dd19a717ed989b419b1e6cb2ac143b
introduced 'existing' keyword to stack-udpate command that became effectively
required argument to stack.update method of the client's Python API,
breaking backward compatibility.

Change-Id: Icda3313805c29ab36773a4edc8c9f8c874b71ca3
Closes-Bug: #1371167

heatclient/v1/stacks.py

@@ -122,8 +122,7 @@ def create(self, **kwargs):
     def update(self, stack_id, **kwargs):
         """Update a stack."""
         headers = self.client.credentials_headers()
-        method = 'PATCH' if kwargs['existing'] else 'PUT'
-        del kwargs['existing']
+        method = 'PATCH' if kwargs.pop('existing', None) else 'PUT'
         resp, body = self.client.json_request(method, '/stacks/%s' % stack_id,
                                               data=kwargs, headers=headers)