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 
probably better to just embed a juju.Context interface and leave it nil -- that'll panic if you try to use unexpected methods, which is good for tests