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.
Then it shouldn't tell us it's failed -- it might happily block for a bit, but it hasn't *failed* while it's still retrying. When it's run out of attempts, sure, it fails: "I tried 5 times, still didn't work, sorry".