wordpress: security upgrade: 4.7.2 -> 4.7.3 & other improvements (#23837)

* Moved the wordpress sources derivation to the attribute pkgs.wordpress. This
  makes it easier to override.

* Also introduce the `package` option for the wordpress virtual host config which
  defaults to pkgs.wordpress.

* Also fixed the test in nixos/tests/wordpress.nix.

nixos/tests/wordpress.nix

@@ -10,14 +10,10 @@ import ./make-test.nix ({ pkgs, ... }:
     { web =
         { config, pkgs, ... }:
         {
-          services.mysql.enable = true;
-          services.mysql.package = pkgs.mysql;
-          services.mysql.initialScript = pkgs.writeText "start.sql" ''
-            CREATE DATABASE wordpress;
-	    CREATE USER 'wordpress'@'localhost' IDENTIFIED BY 'wordpress';
-            GRANT ALL on wordpress.* TO 'wordpress'@'localhost';
-          '';
-
+          services.mysql = {
+            enable = true;
+            package = pkgs.mysql;
+          };
           services.httpd = {
             enable = true;
             logPerVirtualHost = true;