Set a floor on the python-memcache library

python-memcache prior to version 1.48 will leak memory due to always
caching cas ids (compare and set) and with nothing designed to clean
it up.  In 1.48 it was changed to require a boolean argument 'cache_cas'
on init to enable the cas id caching.  In the case of keystone, we rely
on the cache_cas argument and cas functionality.  Ideally, all uses of
memcache should use later than 1.47 to prevent the memory leaking of
cas id caching.

Change-Id: Ide780e30a8243cd5fcc693515ffd84dfea3a3467
Merge "Set a floor on the python-memcache library" into stable/havana