http://mirrors.cloud.aliyuncs.com/centos/6/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 6 – Couldn’t resolve host ‘mirrors.cloud.aliyuncs.com’

是因为已经找不到这个版本的文件了，自然就报错了；将配置文件改为centos7就能找到了

cd /etc/yum.repos.d
vi CentOS-Base.repo
:%s/$releasever/7/g  #将文件中$releasever全部改成7
yum clean all 
yum repolist