Add instructions to disable firewall on Fedora.

https://clusterhq.atlassian.net/browse/FLOC-1007
> 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.

The other option that I considered was to duplicate these instructions in the Rackspace and digital ocean sections. Except that I realized that a generic install probably needs this too.