Use Keystone client session.Session

This replaces the restapi requests wrapper with the one from Keystone client so
we can take advantage of the auth plugins.

As a first step only the v2 and v3 token and password plugins are supported.
This maintainis no changes to the command options or environment variables.

The next steps will include reworking the other API client interfaces to
fully utilize the single auth session.

Change-Id: I47ec63291e4c3cf36c8061299a4764f60b36ab89
Merge branch 'fix-secgroup-add' into dt-test
Merge branch 'rework-restapi' into dt-test
Merge branch 'ksc-session' into dt-test
Merge branch 'identity-ec2-tests' into dt-test