Fixed fqdn handling on debian/ubuntu

The issue was that the old method simply didn't work.
This is what I expect the hosts file to have in it:

    127.0.1.1   new.fqdn.com new

This is what it was after set_hostname:

    127.0.1.1   new.fqdn.com new old.example.com old

Or this if the user didn't set an fqdn in the Vagrantfile:

    127.0.1.1   host host old.example.com old

This patch fixes that.