错误类似于：

Loaded plugins: fastestmirror
appnode | 3.0 kB 00:00:00

http://dl.appnode.com/stable/centos/7/agent/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 503 - Service Unavailable
Trying other mirror.
http://dl.appnode.com/development/centos/7/agent/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 503 - Service Unavailable
Trying other mirror.
http://dl.appnode.com/stable/centos/7/ccenter/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 503 - Service Unavailable
Trying other mirror.
http://dl.appnode.com/development/centos/7/ccenter/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 503 - Service Unavailable
Trying other mirror.