Categorize the trove-manage commands

The trove-manage commands are growing and there are sets of them that can be categorized together so that they not all top level commands.

Nova, Cinder and a couple other projects already have commands categorized. For example nova-manage service, db, host...

Currently we could have trove-manage db, and datastore categories. There will soon be capabilities and configuration-parameters. This will make it easier to list each of the commands out.

Affects:
devstack/lib/trove
trove-integration/scritps/redstack
trove/cmd/manage.py

Example output:
https://gist.github.com/cp16net/9278846