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.
eg we could store a map[k]v in the command, and just loop over it here. (I think there's a SplitKeyValue, or kvSplit, or something func lying around for just this purpose already...)