Support of "default_server" NGINX config attribute

Here I simply give the ability to the Homestead.yaml file to set a default server. I was looking for a solution to use the vagrant share command. Vagrant share was working fine but as there is one unique domain name for multiple sites on my Vagrant box, NGINX was using the first server coming in the list by alphabetical order. With the "default_server" directive in the config file, I'm able to select which server would be used by default when I hit the box.