Adds support for passing extra tenant attributes to keystone

Patch adds simple kwargs parsing to tenant update and create
methods allowing users to exercise the extras functionality
of keystone.

As-per keystone data model docs:
http://docs.openstack.org/developer/keystone/architecture.html#data-model

Change-Id: I1c375c70a0ab4519626d2238d65fa0695f953dc5
Fixes: bug #1180317
Merge "Adds support for passing extra tenant attributes to keystone"