Update OSC's CommandManager subclass

cliff.commandmanager.CommandManager gained an option, update
openstackclient.common.commandmanager.ComamndManager to match.

Also add CommandManager.get_command_groups() to return a list of the
currently loaded command groups.  I expect this to be useful in
upcoming client diagnostic commands for plugins/extensions.

If these turn out to be generally useful we'll propose them to
upstream cliff.

Change-Id: Ic15a7ca0ef975ca679e753be861be7c628b8e10c