Add instructions to disable firewall on Fedora.

https://clusterhq.atlassian.net/browse/FLOC-1007
Perhaps say firewalld here as well.  AWS can indeed have a firewall, it's just outside the instance (and the user already got past the part of the instructions that told them to make sure nodes can talk to each other - although we only discuss SSH elsewhere, perhaps that's an oversight).

It also feels like an abstraction fail to have an AWS section followed by a generic Fedora 20 section which talks about AWS.  I think I'll try to stop thinking about that, though, at least for now.