Add 'command list' command

* Add method to CommandManager to retrieve command names by group
* Add ListCommands

To list command groups loaded by cliff

Change-Id: I37fe2471aa2fafa8aa223159452d52b1981021d6
Merge branch 'review/matthieu_huin/auth_plugins_support' into dt-test
Merge branch 'command-manager-cliff-1.6.1' into dt-test
Merge branch 'command-groups-stuff' into dt-test