Backport #12109

Updated by Loïc Dachary over 6 years ago I think this is doable with the existing teuthology upgrade setups. Similar to the client-upgrade suite, upgrade the cluster, leave one client on the old version, and have another client upgrade.

With a couple scripts (one to act as lock holder, another to act as the proxy requester) we can test that the proxied action worked as expected in both cases (old lock holder + new proxy, new lock holder + old proxy).