Fixes Heat tempest issue due to hardcoded qcow2 extension

The orchestration image_ref is set incorrectly if the extension is not
qcow2, as a result tempest cannot find the Glance image and the
associated tests fail.

This patch fixes the issue by handling any extension.

Change-Id: I32ffe021714590a1b1bab232b1d4f5da238bd4d8
Closes-Bug: #1402774
Merge "Fixes Heat tempest issue due to hardcoded qcow2 extension"