Add API endpoint for overcloud Keystone

The logic of querying the overcloud entrypoints are specific to the
TripleO/Tuskar Heat templates. This API endpoint provides a way for the client
to be independent of this logic.

Change-Id: I552d007d4e1bd3a7558d16138ef96a23a25394ea
Implements: blueprint tuskar-api-return-endpoints
Merge "Add API endpoint for overcloud Keystone"