Run script to comment out `Defaults requiretty` in /etc/sudoers

After the VM has booted (but before we run any sudo commands) run a script to comment out `Defaults requiretty` in `/etc/sudoers`.  This script has no effect in the event there's no such line in `/etc/sudoers`.

This is a subset of #29, but it doesn't try to create a non-root user, it just makes sudo work on distros with `requiretty` turned on out of the box.

This functionality is enabled by default.  I thought about leaving it turned off unless explicitly enabled, but I really can't imagine an instance in which you want a vagrant box with `Defaults requiretty`, so it runs unless disabled.