centos-cert: safer permission handling for .centos.cert
    
    Prior to this change, there was a small window of time where the
    .centos.cert file would be stored with the default umask (eg. 0644 on
    Fedora). This occurs between the time we close the file and the time we
    chown it.
    
    Open the file handle with the 0600 permissions immediately, so the file
    is never world-readable.