Initial step to clean up db and mq attributes

Here we move db and mq attributes already present in default.rb
attributes into new files (database.rb,messaging.rb).  Additionally, we
pull in rabbit/mq attributes from
openstack-{block-storage,compute,image,metering,network} while
transforming namespace from openstack.SERVICE.mq and
openstack.SERVICE.rabbit to openstack.mq.SERVICE
and openstack.mq.SERVICE.rabbit.  Lastly, we copy
openstack.SERVICE.db.username attribute from
openstack-{compute,dashboard,identity,image,metering,network} while
transforming namespace to openstack.db.SERVICE.username.

Change-Id: I50c900aa2ba92ce53f4a6ad05aeabd59e701b514
Implements: blueprint clean-up-attr-for-mq-and-db
Merge "Initial step to clean up db and mq attributes"