Adds Support for Separating Memcached from Horizon

Previously, memcached would always be included in the
openstack::horizon class.  Additionally, the cache_server_ip
parameter was used concurrently by memcached and horizon classes.
This prevented users from 1. Using an existing memcached deployment
2. Binding memcached to a local address and having
Horizon use an external memcached server.

This change implements the configure_memcached and memcached_listen_ip
parameters.  configure_memcached allows users to enable/disable
the memcached class on an as-needed basis.  memcached_listen_ip allows
users to seperate the IP address used for binding memcached from the
address used by Horizon to access memcached.

configure_memcached defaults to 'true' and memcached_listen_ip
defaults to undef for backwards compatibility and to enable memcached
and have memcached use cache_server_ip for binding an IP.

Change-Id: I5e4f4b5786d5ce7da59004a6a55b675a6dd32541
Merge "Adds Support for Separating Memcached from Horizon"