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.
In the case of the Actions spec parsing, the reason this technique was necessary was because nested YAML maps could have interface{} keys, which makes bson.Marshal barf.  In this case, our keys will always be strings, so it's a nonissue until we unmarshal YAML maps directly.