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.
action-set multiple invocation tests and panics fixed
action-set: a jujuc command for Actions to send values back to state

Addressed fwereade's concerns for PR 415.
action-set: a jujuc command for Actions to send values back to state

This commit moves the Action results update method into the Context
implementation, so that client code is not changing the results map
on its own.
action-set PR tweaks

Addressed:
 - need to keep Action results map segregated from jujuc
 - testing Context interface mock to support this change
 - removed old Context methods
 - changed actionResults inner "error" type to string rather than error
Now tests action results interleave in context