Don't download nginx or try to compile it if it's already been installed (from source)

[boosh]

Running this on a vagrant, the playbooks download nginx sources to /tmp. Vagrant destroys /tmp on reload, so now I have to sit through the nginx playbooks downloading everything again since they don't check whether the desired version of nginx has already been installed. It'd be a similar story on an EC2...

It'd be great if the plays were more idempotent and didn't repeat themselves like this.

Thanks!