Allow auth_version in auth_token middleware

Allows the auth_version to be only included when a node
attribute is set to v2.0. This ensures that Keystone v3 works
with Nova when not on Keystone v2.0.

Also corrects the auth endpoint to the main endpoint not
the admin one.

Fixes LP #1204627

Change-Id: Ib7c3eae117a33cc2a7a5668e033851acaecd880b