centos7 yum修改为国内源

修改为阿里源：

sudo mkdir /etc/yum.repos.d/backup
sudo mv /etc/yum.repos.d/*.* /etc/yum.repos.d/backup

sudo wget -O /etc/yum.repos.d/CentOS7-Aliyun.repo http://mirrors.aliyun.com/repo/Centos-7.repo
sudo wget -O /etc/yum.repos.d/epel-7-Aliyun.repo http://mirrors.aliyun.com/repo/epel-7.repo

sudo yum clean all
sudo yum makecache