[HA Guide] Indicate cluster latency requirements
[HA Guide] Indicate cluster latency requirements

Requirements are taken from
https://access.redhat.com/solutions/41070 and viewable without
logging in.

Change-Id: Ie3455cf37a85fd5c60e9c8baab6284972a427a7f

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals