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.
This code would all still be fine -- it'd just move inside the ActionUpdateResults method.

And then we could test that by itself; and we could test the command by itself against a mock of the interface, and just check that the args get parsed cleanly.

(You'd still want ActionSetResults ofc for the overwrite-from-file mechanism)