Ensure constant order when setting all image tags

The newer version of tox (>=1.7.0) sets a random hash seed, which makes the
order of sets unpredictable. This patchset fixes the test_image_tag_set_all
test, which otherwise broke with a randomized PYTHONHASHSEED. This new tox
behaviour is consistent with python (>=3.3), meaning this change should avoid
bugs on that platform at a later date.

Change-Id: Ie281b04fc9c4c2a96a9eec9bde7b05229ebf57fc
Partial-bug: #1348818
Merge "Ensure constant order when setting all image tags"