CentOS repositories for Docker

Summary:
To get an up-to-date version of Docker, the virt7 CBS repositories
offer us the last Project Atomic packages.

This change adds the following repositories to Yum config:

• virt7-common-testing
• virt7-docker-common-testing

Test Plan: make install

Reviewers: Kaliiixx

Maniphest Tasks: T745

Differential Revision: http://devcentral.nasqueron.org/D283