Cleanup cache layer tests

Simple cleanup of keystone/tests/test_cache.py.  Added the following
tests:

* Global Should Cache Toggle
* Configure Non-CacheRegion object raises Validation Error
* Cache Debug Proxy doesn't mangle data / raise exceptions
* Cache Noop Backend, in-fact, does nothing.

Isolation proxy un-used methods removed.

closes-bug: 1218743
Change-Id: Id980a9bff554c72a02461cecbcb533466225a701
Merge "Cleanup cache layer tests"