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.

DEPENDS: #617, #651 
this feels like it's happening at the wrong layer. can't we straight-up fail (or better yet not even expose the tool!) when it's not an action? accepting the request but then ignoring it is not good.