Allow user to specify image to use during rescue

This commit includes the compute changes to accept a parameter
rescue_image_ref in the rescue_instance method.
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: I59311fcfcb8eb54701618e92e5cdada109235b38
Merge "Allow user to specify image to use during rescue"