centos7 更换yum源
centos7貌似没有预先安装wget，所以先安装wget
yum install wget
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
yum makecache如不先安装wget
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
curl http://mirrors.aliyun.com/repo/Centos-7.repo >> /etc/yum.repos.d/CentOS-Base.repo
yum clean all
yum makecache
yum install wget
yum install -y telnet