centos中将yum源换为国内源

1. 备份旧的配置文件
   mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
2. 下载新的 CentOS-Base.repo 到 /etc/yum.repos.d/，并替换部分字段（非阿里云机器需要做）

   wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-8.repo
   sed -i -e '/mirrors.cloud.aliyuncs.com/d' -e '/mirrors.aliyuncs.com/d' /etc/yum.repos.d/CentOS-Base.repo

3. 生成缓存
   yum clean all && yum makecache