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
action-set uniter test added

action-set now has a test in worker/uniter/uniter.  This test
demonstrates how action-set overwrites existing values in the
results map.
ActionFinish refactor fix commit

This commit revises action-set behavior to interact correctly with new
ActionFinish logic on worker uniter.
Add the backups list method.
Add the backups API endpoints for list.
Add the API client methods for list.
Add the backups list subcommand.
Fix gocheck imports.
Adjust to baseSuite.
Return an error for a failed type assertion.
Add a missing doc comment.
Fix a typo.
Improve an error message.
Actions :: Client API
ActionFinish refactor fix commit

This commit wraps up the changes to `action-set` now that ActionFinish
has landed in core.