Bring RESTApi closer to ithe imminent keystoneclient.Session

Prepare to use the (soon to be) common Session from keystoneclient
* Rework RESTApi to eventually be a subclass of keystoneclient.Session

Change-Id: I68e610f8b19a3f6267a93f7bf3de54a228be68aa
Merge "Bring RESTApi closer to ithe imminent keystoneclient.Session"