Wrong os_distro property set for resource

[Lingxian Kong]

distil will set wrong 'os_distro' attribute for resource in the following scenario:

1. boot a vm from a windows volume
2. rebuild vm using a linux image
3. nova won't change operating system, but will set image_ref inappropriately.
4. distil will get os_distro info from image_ref, then from bootable volume information.

Seems Nova also has a bug, but distil should be enhanced to avoid the problem.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/417714

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/417714
Committed: https://git.openstack.org/cgit/openstack/distil/commit/?id=a9d9b759c6f1168c73b59ec6c08e1b369698ecd1
Submitter: Jenkins
Branch: master

commit a9d9b759c6f1168c73b59ec6c08e1b369698ecd1
Author: Lingxian Kong <email address hidden>
Date: Fri Jan 6 22:17:43 2017 +1300

    Fix getting 'os_distro' property of instance

    When VM is booted from a windows image and do a rebuild using a linux
    image, the 'image_ref' property will be set inappropriately. This fix
    will look for if VM is booted from volume first, then check image
    property afterwards.

    Change-Id: I3e6026787f15209b8db8a1db801e5e076dc52501
    Closes-Bug: #1654417