Enable Memcache service for Token Caching

With the release of 4.2.0 of keystonemiddleware using the
in-process token cache is no longer recommended. It is recommended
that a memcache backend to store tokens is used instead,

This installs and configures memcache and configures neutron-server
to use memcache for token caching.

http://docs.openstack.org/releasenotes/keystonemiddleware/mitaka.html#id2

Change-Id: I1f7764afb24b511508432fa758596cc9ab40aba8