add diskspace to isobuilder1-4

Originally created by @groente on #14732 (Redmine)

the isobuilders are running low on diskspace and i can’t see any non-essentials eating up significant amounts of diskspace, so i’m tempted to give them all 1gb extra on vg1-root, what do you think?

Related issues

• Related to #13425 (closed)
• Blocks #13233 (closed)