Prevent log collection from growing to astronomical sizes

The delete_instance.sh script collects logs as part of the deletion process. However, if things went wrong somehow, it seems to collect from the wrong place (see e.g. https://gitlab.cern.ch/cta/CTA/-/jobs/45531244/viewer). Specifically from / instead of from /var/log.

As a result, the logs can grow extremely large. This should be fixed in two ways:

• Fix the bug that causes it to read from /
• Prevent the collected logs from being more than 1GB