Override path of yum repo if rhel or centos 7.

Override path of yum repo if rhel or centos 7.

In current realisation it doesn't work with rhel7 and centos7:
http://repos.sensuapp.org/yum/el/7/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found

Maybe it will be fixed later, when packages for 7 release will be available?