nginx: added serverName option for virtualHosts

This allows overriding the `server_name` attribute of virtual
hosts. By doing so it is possible to have multiple virtualHost
definitions that share the same `server_name`. This is useful in
particular when you need a HTTP as well as a HTTPS virtualhost: same
server_name, different port.

nixos/modules/services/web-servers/nginx/vhost-options.nix

@@ -8,6 +8,15 @@
 with lib;
 {
   options = {
+    serverName = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = ''
+        Name of this virtual host. Defaults to attribute name in virtualHosts.
+      '';
+      example = "example.org";
+    };
+
     serverAliases = mkOption {
       type = types.listOf types.str;
       default = [];