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
Add missing requests and six requirements

From global-requirements: requests>=1.1, six>=1.4.1

Change-Id: I536adc511931229a268ba81f81aef1aed59b33eb
Merge "Add missing requests and six requirements"