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.
This is there to ensure conformance to the Actions spec doc.  `--failed` is less about an actual error state than it is about a user-defined "failure" state, for example, the remote server failed to authenticate the given API key for S3.