add support for Horizon's caches backend configuration (memcached)

With this we add into the Horizon element support for two new
keys meant to configure the Horizon's session storage mechanism with
memcached.

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

Change-Id: Ife7137054eafba548d13c58265abd6373679794e
Merge "add support for Horizon's caches backend configuration (memcached)"