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.
don't really understand bare values *anyway*. I don't think there's much call for a result that doesn't say what it is -- if we allow it, then we encourage people to use it, and then they'll have problems when they inevitably discover the need for a second piece of data later...