add Horizon caches backend definition (memcached)

This change adds into the overcloud-source template a structure
name horizon.caches meant to define the Horizon caches backend.
It defaults to using memcached and provides a list of the
memcached nodes in horizon.caches.memcached.nodes

Related to blueprint tripleo-icehouse-ha-production-configuration

Change-Id: I728e05926f2de0e867fb8e8c74c63947da7d987a
Merge "add Horizon caches backend definition (memcached)"