Adds support for Nova Memcached

Previously, the openstack::nova::controller class could only use
Nova's in-process cache for storing information such as
nova-consoleauth session tokens.

This patch adds a new parameter called memcached_servers that
allows Nova to use memcached instead of the in-process cache.

Defaults to false to continue having Nova use the in-process
cache for backwards compatibility.

Change-Id: Ic61962e6ebf588e5724d238954ec611b0bbcbd05
Merge "Adds support for Nova Memcached"