Cache tokens using memorycache from oslo.

Verifying tokens accounts for a significant portion of the time taken
by the various api servers. This uses the simple memory cache from
oslo so that there is a simple default cache even if memcached is not
deployed. Also cleans up the tests and removes unnecessary fakes.

DocImpact

Change-Id: I501c14f2f51da058cb574c32d49dd769e6f6ad86
Merge "Cache tokens using memorycache from oslo."