Extract a base Session object

A wrapper around a number of connection variables. This will be extended
later with principals such as Kerberos authentication and http sessions.

The intent is that this session object will become the basis for all
other client library communications in OpenStack (as keystone wants to
control things like authentication for everybody).

Change-Id: I8ee728c49d554659d7057ebf17d0f8ceea4d7d8e
Part of: blueprint auth-plugins