Hrm, I don't understand how this can work with pebbles:
 - there's no os_install/execute/etc. files in http://$ADMIN_IP:8091/discovery/pxelinux.cfg/ anymore. The only one we have is default.
 - the node-specific file is not using a format like "01-$(echo $MAC | tr '[:upper:]:' '[:lower:]-')" anymore, but something like C0A87C51

I think the right way to do this would be:
 - for the "wait for os_intall" case: wait for the C0A87C51 file to appear
 - for the "wait for execute" case: wait for the C0A87C51 file to disappear