Improved messaging server section
Improved messaging server section

I improved the messaging server section including structure and
content to meet conventions. I also added links to each message
broker for those who may consider using a different message broker
than the one supported by their particular distribution.

Change-Id: Ic046ae7ceee315a9da91f8530542c2e0cb3a66f9
Closes-Bug: #1309304
Closes-Bug: #1298153

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