Update database content for Juno
Update database content for Juno

I updated database content in the installation guide for Juno
as follows:

1) Changed Ubuntu, RHEL, CentOS, Fedora, and OpenSUSE to use
   MariaDB. Debian and SLES continue to use MySQL.
2) Adjusted wording to remove dependencies on specific database
   servers.
3) Implemented changes from the improvements blueprint including
   procedure structure and note about PostgreSQL.

Change-Id: I8608e2329d87b237a259f366976e8ec1b7f50bdc
Implements: blueprint installation-guide-improvements

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