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.

DEPENDS: #617, #651 
I have a feeling I already said this somewhere, but isn't it cleaner to put *all* the action-related stuff in a single *struct on the context? otherwise you've got these various related fields which are valid or not, depending on the values of other fields.