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.
Hold on. I suspect we want to check the values of the key strings for sanity's sake -- and I think we did something very similar in the actions code in the charm package. I think there's a chunk of commonality to be extracted (and maybe slipped into the schema package? `NewNestedStringMap(validKeyRegexp)`?)