Remove redundant tests from Qcow2TestCase

Qcow2TestCase had exact copies of two tests in the parent
_ImageTestCase so remove them. I'm assuming the tests originated
in Qcow2TestCase but later more image types were added and the
tests were pulled up into the common parent mixin since they
applied to all types of images, but the redundant tests from
Qcow2TestCase weren't cleaned up.

Related-Bug: #1302211

Change-Id: I031516b555ead5a3769fb62d90e373aac1b6baf1
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 "Remove redundant tests from Qcow2TestCase"
Merge "Allow user to specify image to use during rescue"