Fix Ubuntu box build script sudoers syntax

When executing sudo -u inside the Ubuntu lxc vagrant box:

$ vagrant ssh
$ sudo -u vagrant ls 

I get this error: `Sorry, user vagrant is not allowed to execute '/bin/ls' as vagrant on <machine>.`

Googling about the message i found a similar issue in the vagrant repository: https://github.com/mitchellh/vagrant/issues/892

The solution is very simple, just a syntax fix in the build script that handles /etc/sudoers file.