Save some disk space on VMs

Fixes #189 (closed).

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This downsizes swapfiles on VMs - since our plan of fixing low-on-memory situations as they occur hasn't worked out very well. We haven't suffered from slow-growing memory leaks so far, only from exploding memory usage on Nix software upgrades, and there the swap usually barely helped regarding making it to the end or be quick enough to fix the issue at hand.

Also this MR caps the disk space Journald is configured to use on machines that send their logs to our central logging server.

Both together should help us doing disk space foraging on VMs less often and so save a bit of time, regularly.