Add auth_plugin support to cinderclient

With CINDER_RAX_AUTH being rightfully removed, cinderclient is no longer
compatible with Rackspace/any non-keystone auth. To fix this, I stole
auth_system/auth_plugin from novaclient's implementation.

See https://review.openstack.org/#/c/23820/.

Change-Id: If5f84003f868ef02bb7eb7da67cf62018602e8f0
Closes-Bug: 1280393
Merge "Add auth_plugin support to cinderclient"