Server block LWRP and nxensite/nxdissite script

Created a LWRP for creating, deleting, enabling and disabling nginx server blocks. And added site enable / disable script written by @perusio
In the original config file you're setting the web root hardcoded to '/var/www/nginx-default', and there is no `node["nginx"]["webroot"]`, or what one might call a web root setting.
[See here](https://github.com/phlipper/chef-nginx/blob/master/templates/default/default-site.erb#L8)

@phlipper: I propose to either create a web root setting and then use that, or leave this as is.