Don't reorder config in /etc/network/interfaces

On Debian the network interface configuration is rewritten, with the Vagrant section appended, every reboot. But the network interface file is order dependent when additional virtual interfaces are added by installed packages (`ucarp` being my motivating example). Moving the Vagrant section to the end breaks virtual interfaces (like `ucarp`'s virtual IP).

This patch changes this so that it effectively rewrites the Vagrant section in `/etc/network/interfaces` in place rather than moving it to the end of the file.

If the section doesn't exist, it still gets appended as before.