WIP: hhvm deploy

(not mergable yet - opening PR only so that I don't forget about this)
Heh, sorry :) The question is a bit misleading, though: I meant "what is the reason" as in: "I do not want to add an additional param to easybib_nginx, but depending on the acutal use case, I might have ideas how to make this a variable without adding another param." Since we already supply `app_name`, you could for example just modify `easybib_nginx` to access node['deploy'][app_name]['asset_root']. This would keep the definition of `easybib_nginx` smaller (its already pretty large), and you could add all sorts of `.nil?` checks for a sane default before. Right now, it would cause a failing deploy if deploy has no such key set.