Unify host naming in readme file
Unify host naming in readme file

As we use 'mariadb', 'rabbitmq' in both cinder config
file and docker compose file, replace the 'dbhost' with 'mariadb'
and 'rabbit' with 'rabbitmq' to eliminate confusion.

Change-Id: I861ce1daa9f216b9f854a9ede520c99a83c9beff

eharney/cinder,mahak/cinder,openstack/cinder,j-griffith/cinder,openstack/cinder,Datera/cinder,Datera/cinder,eharney/cinder,mahak/cinder,phenoxim/cinder,j-griffith/cinder,phenoxim/cinder