What OS should a master node CI virtual machine be?
I think the major deciding factor is the expertise of you and the rest of your company in the chosen OS. If you are a Windows shop, and your company is willing to leverage the cost of the node, it's probably best choice to host it on a Windows VM. For choosing a Linux distro, I would see what is most common across your company. In my opinion, it is not worth the benefit of creating a vm with a one off distro just because of a few particular features, unless those features of the distro are absolutely necessary.