geemus suggested that we can selectively load providers from fog, cutting down the amount of code we load.

https://github.com/fog/fog/issues/3295#issuecomment-64659051

app/models/concerns/fog_extensions.rb mostly does this already, but does a require 'fog' at the top, as does ssh_provision.rb, and bundler will require the gem by default. These could probably all be removed.

We should ensure though that any plugins using Fog (xen, one, docker(?) etc) all have the appropriate requires in place to ensure their providers are accessible.