Rework keystone auth_token middleware configs

We're using Sahara-specific configs and there are some other potential
inconsistencies.

* use common OpenStack [keystone_authtoken] section for middleware
  configurations;
* token validator reworked to be consistent with update auth_token
  middleware usage;
* auth_uri is now stored in context for consistency, additonally, it
* provides correct auth_uri in multi process sahara deployment.

Closes-Bug: #1257472
Closes-Bug: #1249063

Change-Id: I5a33ae6269d40dadcd4893b27a937a37e0c74006
Merge "Rework keystone auth_token middleware configs"