Add RH-family support

This PR consists of two commits but the first has already featured a previous PR and when that is merged, then this one will only amount to the second commit.
yum-epel is a cookbook available in the chef supermarket, so everyone has it.
In many cookbooks I install chef downloads windows stuff just because the cookbooks supports windows, but those never run on Linux architectures.
In the same way, the yum-epel cookbook will only run on centos, not debian (see line 20 of recipes/default.rb)