Restructure how Nginx parser re-finds vhosts, and disable creating new server blocks. (#3528)

* Restructure add_server_directives to take a vhost as argument. This is the first step towards fixing vhost selection in nginx.

* Save path to vhost in file while parsing in get_vhosts().

* Disable creating a new server block when no names match.

* Make parser select vhost based on information in the vhost it found previously, rather than searching again for a match.

* Make add_server_directives update the passed vhost

* Update boulder config to pass test

* Add testing code for the _do_for_subarray function

* documentation and formatting updates

certbot-nginx/tests/boulder-integration.conf.sh

@@ -52,6 +52,7 @@ http {
     listen 8081;
     # IPv6.
     listen [::]:8081 default ipv6only=on;
+    server_name nginx.wtf;
 
     root $root/webroot;