Sync oslo-cache from oslo-incubator

This patch removes Marconi's local copy of the oslo-incubator cache
work. The patch also removes the memcached backend, although the version
coming from oslo doesn't have support for it yet. The backend will be
pulled in again as soon as it is updated upstream.

Change-Id: I487b2178d9657eb7abe020f06436b14531bdb993
Merge "Sync oslo-cache from oslo-incubator"