Drop auth_token configs for api-paste.ini.

We now prefer to use nova.conf to configure keystoneclients
auth_token configs.

Also, Devstack has now been update to allow this:

  https://review.openstack.org/#/c/52258/

Change-Id: I2a4b300af37eddf5010b97ca796f0552941642a8
Merge "Drop auth_token configs for api-paste.ini."