Fix cache configuration checks

There were two different checks to see if a cache region was already
configured with a backend. Both were changed to use the
Region.is_configured property.

The initial reason for this change was that the hasattr approach in the
test fixture was causing some issues in Python 3. For some reason it was
forcing the __get__ lookup on the descriptor and Python 2 did not.

bp python3

Change-Id: I409656f5ede08160cc68d48e7a3a636328e3e9a6
Merge "Fix cache configuration checks"