API extension for accessing instance_actions

Adds a new API extension for accessing the recorded actions and events
on an instance.  Usage is documented with api samples.

Additionally it modifies the db api for retrieving events by
instance_uuid and request_id since the api does not return the action id
and so events can not be retrieved based on that.  This extension is the
first consumer of those db method so there's no issue of changing
behavior elsewhere.

Blueprint instance-actions
DocImpact

Change-Id: I74109586cc762a7f51d2b114896cf071ee0671cb