Deprecate admin_token option in auth_token

The admin_token option shouldn't be used with the auth_token
middleware. It's used to specify a token to be used to perform
operations on the identity server, so would typically be set
to the admin token. The admin token should only be used to
initially set up the Keystone server, and then the admin token
functionality should be disabled. If this recommended setup is
used then the auth_token middleware shouldn't be using the
admin token / auth_token.

In preparing for removal of the admin_token option, the option
is now deprecated. A warning will be logged if it's set.

DocImpact

Change-Id: I5bc4f4a6ad7984892151c8011ccd92f166aba4c2
Closes-Bug: #1306981
Merge "Deprecate admin_token option in auth_token"