/etc/yum.repos.d目录全部移动到tmp文件夹，自己新建

yum clean all

yum --disableplugin=fastestmirror makecache

yum --disableplugin=fastestmirror update


 rpm -ivh --replacepkgs --replacefiles centos-release-7-6.1810.2.el7.centos.x86_64.rpm

    rpm -Uvh https://mirror.webtatic.com/yum/el7/epel-release.rpm

    rpm -Uvh https://mirror.webtatic.com/yum/el7/webtatic-release.rpm