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.
whoa! *definitely* not: merely calling a command in an idiotic way is not a good enough reason to mark the action failed -- the only reasons we should fail are:

 - explicit `action-fail`
 - the action script returning a non-zero exit code

It's the author's choice how to handle the failure of any tools they happen to use.