Enforce credentials requirement on stack create/update.

Enforce credentials requirement on stack create/update.

Credentials need to be supplied whenever they are stored with the stack,
which is during stack create and update.

Some users have been relying on their auth token to perform all heat
operations (as all other openstack services support) however this has
led them to experience obscure errors when wait handles or alarms are
triggered.

Failing early with a clear error message will stop this class of errors.

Fixes bug: #1194303

Change-Id: I3edef3dee843bb06760be6294b798761eba30cb8