heat_keystoneclient add legacy fallback path

If users fail to observe the reccomendations in the commit messages
and/or release notes and use a heat.conf from a previous version of
heat currently some functionality will fail due to the missing
domain configuration in heat.conf.  Instead degrade gracefully with
warnings, calling the non-domain functions which are maintained
(for now) for backwards compatibility.

Hopefully we won't have to maintain this huge pile of legacy stuff
for too long :(

Change-Id: Ia029d585dfdb2563609725589c41083819a11a3b
Closes-Bug: #1287980