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 
wait, are these a load of test-specific global variables? unless they're shared, please put them inside the tests that use them -- I have sadly concluded that humanity as a whole just cannot be trusted with global variables, even in tests,