Parametrize hosts

Allows to use mailu without docker-compose when hostnames are not set up
by docker itself but provided via a separate resolver.

Use case: use mailu using nomad scheduler and consul resolver instead of
docker-compose. Other servers are provided by the DNS resolver that
resolves names like admin.service.consul or webmail.service.consul.
These names needs to be configurable.

core/nginx/conf/nginx.conf

@@ -37,10 +37,10 @@ http {
     # Main HTTP server
     server {
       # Variables for proxifying
-      set $admin admin;
-      set $antispam antispam:11334;
-      set $webmail webmail;
-      set $webdav webdav:5232;
+      set $admin {{ HOST_ADMIN }};
+      set $antispam {{ HOST_ANTISPAM }};
+      set $webmail {{ HOST_WEBMAIL }};
+      set $webdav {{ HOST_WEBDAV }};
 
       # Always listen over HTTP
       listen 80;