fix centos install issue

A new naming convention was put into place for CentOS 7 that breaks how ceph-deploy picks up correct naming conventions.

This allows installation of CentOS again for version 7

![screen shot 2014-07-14 at 12 24 17 pm](https://cloud.githubusercontent.com/assets/317847/3573550/5a9d9be4-0b73-11e4-820c-6c0e0aa39c74.png)