clean up modify_fstab
I don't see any reason why modify_fstab can't run on any machine; true, it's not necessary to preserve fs mount options on VMs that will be destroyed before they are rebooted, but ... someone might reboot the VM without destroying, and editing fstab should not be harmful, so the special case is unnecessary.
Figure out why this was deemed necessary and justify it better or remove it.
Other outstanding weirdnesses around this code:
1) it's crazy Perl; if the only reason we need the Perl is because we need
the mountpoint and the UUID, as the comment in roles/testnode/tasks/apt_systems.yml claims, those should be available through facts.
2) why is this for apt systems only? If it's necessary, surely it's necessary regardless of the package type of the system?