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
74a27056b3