[nginx] set max_body_size as variable with default

roles/nginx/defaults/main.yml

@@ -2,3 +2,5 @@
   is_proxy: false
   php: false
   config_name: null
+  # max body size in Mb
+  max_body_size: 8

roles/nginx/templates/proxy.conf.nginx.j2

@@ -1,5 +1,5 @@
     location / {
-        client_max_body_size 50M;
+        client_max_body_size {{ max_body_size }}M;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection "upgrade";
         proxy_set_header Host $http_host;