Disable RequireTTY in /etc/sudoers

...to avoid the following error:

+ sudo -u stack /opt/stack/install-devstack.sh
sudo: sorry, you must have a tty to run sudo
Remove all untested or non-compatible distros

To be re-added as they are tested.
Merge pull request #3 from jasondunsmore/master

Fix requiretty issue with devstack, reduce distros to CentOS 6.5