Fix heat-api-cfn 500 error on an unknown action

Add a default function in heat-api-cfn StackController, and raise
an HeatActionNotSupported exception.

Change-Id: I9023817792e949bde558dd22e818f38d6185f0fd
Closes-Bug: #1473083

heat/api/cfn/v1/stacks.py

@@ -48,6 +48,9 @@ class StackController(object):
         self.rpc_client = rpc_client.EngineClient()
         self.policy = policy.Enforcer(scope='cloudformation')
 
+    def default(self, req, **args):
+        raise exception.HeatInvalidActionError()
+
     def _enforce(self, req, action):
         """Authorize an action against the policy.json."""
         try:

heat/tests/api/cfn/test_api_cfn_v1.py

@@ -62,6 +62,10 @@ class CfnStackControllerTest(common.HeatTestCase):
                                                        'deny_stack_user.json')
         self.addCleanup(self.m.VerifyAll)
 
+    def test_default(self):
+        self.assertRaises(
+            exception.HeatInvalidActionError, self.controller.default, None)
+
     def tearDown(self):
         super(CfnStackControllerTest, self).tearDown()
         cfg.CONF.unregister_opts(self.opts)