[RHEL/6/kickstart] Rename usgcb-server-ks.cfg to usgcb-server-with-gui-ks.cfg to detail
                   it will create Red Hat Enterprise Linux 6 Server with GUI instances as
                   a result. Also add a note explaining why use of latest Git repository
                   content should be preferred for now rather than use of RHEL-6 benchmark
                   from native RHEL-6 scap-security-guide RPM package.