引用
redhat-upgrade-tool --force --network 7.0 --instrepo http://centos.communilink.net/7.3.1611/os/x86_64/
使用--force选择，强制升级
参考： http://blog.hlogc.com/20160313/upgrade-from-centos-6-to-centos-7/
centos7下载地址： http://mirrors.gigenet.com/centos/7/isos/x86_64/
http://isoredirect.centos.org/centos/7/isos/x86_64/CentOS-7-x86_64-Everything-1611.iso
