heat_keystoneclient add get_user_token

Add a function which takes a stack domain username/password and
returns a token - this is for "one time" use, e.g for callback
signals to wait conditions via the native API.

blueprint: native-waitcondition

Change-Id: I6f4666728c5ecadb1d9d081fb825a77cd598fd47
Merge "heat_keystoneclient add get_user_token"