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 is not testing UpdateActionResults, it's testing action-set. (which might call UAR under the hood, or it might not -- who knows? ;p)