For Milestone 1, we need to implement the fact the evergreen-client upgrades Jenkins.

From a high level, this should be done as follows:

1. The client is instructed to perform an upgrade (Not sure: I think this is JENKINS-50848)
2. It downloads what needs to be (plugins, new war) and puts them at the expected place for next startup (see below)
3. Restart Jenkins to pick-up update
4. Health-check it, see JEP-304
5. If unhealthy, roll it back to previous, see JEP-302

Open questions

• What if the current state, i.e. even before upgrading, is unhealthy already?
• "expected place for next startup": we should probably define some location on the disk for caching those things, for instance to be able to revert in case we need to rollback without downloading things again, etc.