[install] Note the memcached service runs on the controller node
[install] Note the memcached service runs on the controller node

backport:mitaka

Change-Id: I5ac25b01bd77191de603f16eef8ae0628fbc3957
Closes-Bug: #1588619

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals