sudo requires a tty to function by default on centos. Instead of
tweaking the sudo conf we can just add a tty. This has the added
advantage of making the containers more friendly if you have to
`docker exec -i <container> bash` into them.

Change-Id: If97a02ca1d37c243a787d98ade54bde8d641aecd
backport: liberty
Partially-Implements: blueprint functional-testing-gate