In the code for handling rolling upgrades (and pinned ironic versions),
we were assuming that for an object class (eg 'Node') that had more than
one version in a release, the first version was the latest one. This was
error-prone as humans don't always know/remember to put the latest version
as the first one in the list.
This patch removes the assumption and gets the maximum version in the list.
Change-Id: Ie0a789f8bfa6f286b58cc7211be85ee18b2800f5