Make "quantum help" to show a list of subcommands.

Fix bug 1023260

All other OpenStack clients (nova, keystone, glance, ....) accepts
just "help" which displays a list of subcommands. This commit makes
quantum command consist with other OpenStack projects.

After this change help behavior of quantum command becomes as follows:
Show general help message:
  quantum --help
  quantum help
Show help message of subcommand
  quantum help <subcommand>
  quantum <subcommand> --help

Change-Id: I34ca0df809da04f9b9b9275c697e6aafca312cfe
Merge "Make "quantum help" to show a list of subcommands."