replace manualNixMachines by etcFilesProvidedByAdmin

Then you can also maintain other etc files such /etc/hosts yourself.
Merge branch 'submit/manual-etc-hosts' into systemd/marc

* submit/manual-etc-hosts:
  replace manualNixMachines by etcFilesProvidedByAdmin

Conflicts:
	modules/system/etc/etc.nix