COOK-4323 add nginx site template


- Added documentation & serverspec tests for `nginx_site` definition :smile_cat: 
- Removed template from `nginx::commons_conf`, `nginx_site` now handles setting up the default site
- Add sane defaults to `default-site.erb` template
- Made `default-site.erb` template flexible with custom parameters & templates. (Modeled after apache2 cookbook's `web_app` definition)
- Fixed default(_server) directive for nginx versions < 0.8.21