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.
Missing the obvious case: `action-set ping=pong`

I think we may need some more complex examples too. What happens when a map overwrites a value, or vice versa?