General info added to HA guide intro
General info added to HA guide intro

Now that the intro covers more than just active/passive,
this patch adds information on stateful vs stateless
components and active/passive vs active/active.

bug #1196096
blueprint improve-high-availability-support

Change-Id: Ibee6e07ba240ac37ea0770a6aead19fc810f58ea

openstack/openstack-manuals,saeki-masaki/openstack-manuals,Akasurde/openstack-manuals,potsmaster/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual,potsmaster/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,kairoaraujo/openstack-manuals,saeki-masaki/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals