Implement upgrade testing for os_glance role.
This patch adds an upgrade testenv, which configures a previous version
of glance, and the test-vars from the stable/newton branch to be used by
the previous version of glance.
We don't run functional tests after the initial stable/newton deploy
because a working stable/newton branch is already tested, and it's
assumed it worked if the upgrade works too - which is more efficient.
This follows the pattern setup for Keystone in:
https://review.openstack.org/#/c/384269/
Change-Id: I8b2418ffc9197814b0eb85da647916323f9cefd5
Implements: blueprint upgrade-testing
6bd22d2528