docs(managing_deis): add note for public cloud environments

refs #2479, as it is a good way to document security concerns in a public setting but requires a larger discussion on the possibility of automating these steps through `deisctl`.
Maybe make the "Therefore,..." sentence its own paragraph--for more emphasis--and phrase it this way?

Therefore, if you run Deis on a public cloud without security group features, set up such rules using `iptables` or a similar tool.