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.
I'm not sure I understand what you don't like about it.  Right now, I have it parsing bare values into a map with default keys "val%d".  I thought that was what you were looking for, so it seems we've crossed wires!