For some Nova scenarios, we will need suppot for
set_multi/get_multi/delete_multi for writing to either
memcache or dictionary backend. So let's add support for
operating on multiple keys. Note that if we don't implement
this the default CacheBackend class throws NotImplementedError
Closes-Bug: #1483322
Change-Id: I1aca9e65b3be038a507ced8dd039c5413d6c4ac2