Add dns A record for mirrors.stg.centos.org

Currently mirrors.stg.centos.org doesn't resolve, but on the fedora side we try and set it up and get a cert for it, causing that playbook to fail. ;(

It would be great if mirrors.stg.centos.org could resolve to wildcard.stg.fedoraproject.org

(This should resolve to our currently active staging proxies).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information