action-set: a jujuc command for Actions to send values back to state

action-set permits action hooks to define an arbitrary map of params
which will be returned to the state server at the end of the Action,
or to specify --failed with an optional failure message.
I'm not sure how an error in the action-set command should be handled, then.  Right now, we're returning nil in order to avoid a Hook error condition.  Therefore, unless we set an Action error (in the context, to be returned), I'm not sure how the user will ever know that their argument failed to unmarshal.