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 
and why do we have "action failed" here? do we have it everywhere? in general, if we want those words in the error messages (not 100% sure we do?), we should be inserting them OAOO.