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.
Yeah, this is too much. We should just be testing this against a simple implementation of the Context interface -- check a single invocation either makes a given Update call, or errors. (or, later, with file input, makes a Set call).

You can do really clean interface mocks by just embedding the interface you want to implement, leaving it nil, and implementing only the methods you expect to call: you don't have to implement loads of do-nothing methods, any unexpected usage will panic nicely.