Refuse invalid qcow2 backing files

Don't allow qcow2 files that are pointing to backing files outside of:

volume-<id>
volume-<id>.<snap-id>
volume-<id>.tmp-snap-<snap-id>

(optionally prefixed with /mnt/path)

Closes-Bug: #1350504

Change-Id: Ic89cffc93940b7b119cfcde3362f304c9f2875df
(cherry picked from commit dca3c8323cf8cf12aa8ce4ba21f647ce631e8153)
Sync latest processutils from oslo-incubator

An earlier commit (Ia92aab76fa83d01c5fbf6f9d31df2463fc26ba5c) failed
to address ssh_execute(). This change set addresses ssh_execute.

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oslo-incubator head:

commit 4990535fb5f3e2dc9b397e1a18c1b5dda94ef1c4
Merge: 9f5c700 2a130bf
Author: Jenkins <jenkins@review.openstack.org>
Date:   Mon Sep 29 23:12:14 2014 +0000

    Merge "Script to list unreleased changes in all oslo projects"

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The sync pulls in the following changes (newest to oldest):

6a60f842 - Mask passwords in exceptions and error messages (SSH)

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Change-Id: Ie0caf32469126dd9feb44867adf27acb6e383958
Closes-Bug: #1377981
(cherry picked from commit 5e4e1f7ea71f9b4c7bd15809c58bc7a1838ed567)
Merge "Refuse invalid qcow2 backing files" into proposed/juno