Not enough space in /var/lib/jenkins on isobuilders

Originally created by @intrigeri on #12595 (Redmine)

This ticket is about two issues:

Short term: noise on our monitoring dashboard panel/notifications. Under normal conditions we use up to 20GB out of 23GB there, and our monitoring doesn’t allow that.
Medium term: we lack disk space in /var/lib/jenkins on isobuilders. Once #12576 (closed) is done we’ll be hosting multiple baseboxes in /var/lib/jenkins/.vagrant.d/ so we will need even more space there, so this disk space issues is currently blocking such performance optimizations. See #12531 (comment 47187) and follow-ups where the research about how many baseboxes we need to store there is being worked on.

Edited May 15, 2020 by intrigeri

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