[horizon] memcache backend not configured for horizon

[odyssey4me]

When using horizon in an HA environment, sessions are not properly retained across horizon instances as the memcache backend is not configured in local_settings.py.

The recipe should check if memcache is included in the horizon server's recipe list (or an attribute could be set to make horizon use the memcache backend), then it should enable https://github.com/rcbops-cookbooks/horizon/blob/master/templates/default/local_settings.py.erb#L82-L90 and disable https://github.com/rcbops-cookbooks/horizon/blob/master/templates/default/local_settings.py.erb#L92-L96