Change occurrences of keystone to identity server

The auth token middleware referred to Keystone everywhere, but
Keystone is only one possible implementation of the identity API.
The code should refer to the identity API or identity server where
appropriate.

Change-Id: I67a0c203a4e0482b36b021e98e8ac99e29b0c2df
Merge "Change occurrences of keystone to identity server"