Add CentOS 7 support for mesosphere install

We're using this cookbook in production at Brigade, but unfortunately we are forced to use `node[:mesos][:type] = 'source'` because we are on CentOS 7 and the mesophere build of mesos for CentOS 6.5 understandably doesn't work on CentOS 7.

It takes a long time to build Mesos, especially given that Mesosphere has done all the hard work for us, we just need to swap out the URL.

This commit adds a simple conditional which swaps out the URL to the mesosphere repo based on the version of CentOS.

Let me know how this looks. Thanks for the great, well-tested, cookbook!