nginx: Allow specifying a server_name to use in /etc/zulip/zulip.conf variables

Some folks deploying Zulip alongside other software on their system find that they need to manually add a server_name directive to our nginx configuration (see reverse diff of what puppet tries to change below):

Notice: /Stage[main]/Zulip::App_frontend/File[/etc/nginx/sites-available/zulip-enterprise]/content:
--- /etc/nginx/sites-available/zulip-enterprise 2017-05-28 22:45:12.457616905 +0545
+++ /tmp/puppet-file20170531-26102-n3g8ux   2017-05-31 22:26:21.809825910 +0545
@@ -1,9 +1,6 @@
 server {
     listen 80;
     return 301 https://$host$request_uri;
-
-    server_name subdomain.redacted.org;
-
 }

 include /etc/nginx/zulip-include/upstreams;
@@ -11,8 +8,6 @@
 server {
     listen 443;

-    server_name subdomain.redacted.org;
-
     ssl on;
     ssl_certificate /etc/ssl/certs/zulip.combined-chain.crt;
     ssl_certificate_key /etc/ssl/private/zulip.key;

Ideally, we'd have this setup so that you can tell Zulip what server_name you're using, and things will just work.

This is potentially related to the Certbot issue I've been looking into.