wordpress: move wordpress_docker_volumes to vars as it should generally not be overridden

roles/wordpress/defaults/main.yml

@@ -7,6 +7,3 @@ docker_additional_services: []
 docker_host_user: false
 wordpress_symlinks: false
 
-wordpress_docker_volumes:
-  - name: html
-    path: /var/www/html

roles/wordpress/vars/main.yml

@@ -0,0 +1,5 @@
+---
+
+wordpress_docker_volumes:
+  - name: html
+    path: /var/www/html