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
Move plugin stuff to clientmanager

The OSC plugins work by adding an object as an attribute to a
ClientManager instance.  The initialization and management of
thos plugins belongs in clientmanager.py.

At this point the only part not moved is the API version dict
initialization bcause the timing and connection to the
CommandManager initialization.  It gets refactored anyway when
API discovery becomes operational.

Change-Id: If9cb9a0c45a3a577082a5cdbb793769211f20ebb
Merge "Add 'command list' command"
Merge "Move plugin stuff to clientmanager"