Allow user to specify image to use during rescue - V2 API changes

This commit includes the V2 changes.
Making changes in v2 API to take an extra parameter rescue_image_ref
as part of the rescue action API.
If specified, this image_ref will be used to rescue the instance.
If the image is not specified, then the base image ref is used by
default.

Partially Implements: blueprint allow-image-to-be-specified-during-rescue

DocImpact

Change-Id: I985b21264841a6a18a66d19ccd41753f78576776
Merge "Allow user to specify image to use during rescue - V2 API changes"