Allow user to specify image to use during rescue - V3 API changes

This commit includes the V3 changes.
Making changes in V3 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: I7a9d88bf4dfe73879247f5b82a52928fdde76510
Merge "Allow user to specify image to use during rescue - V3 API changes"