Remove usage of admin_token in nailgun

Instead of using admin_token in nailgun to verify keystone
tokens, nailgun will have dedicated service user to do that.

Change-Id: I3cf8f43cb03bfe3c523d78f57d2082451187649b
Implements: blueprint access-control-master-node-improvments
Merge "Remove usage of admin_token in nailgun"