Make message_size_limit configurable

pillar.example

 postfix:
   nullclient:
+    message_size_limit: '10240000'
     relayhost: 'smtp.example.com'
     rootmail-destination: 'rootmails@example.com'

postfix/nullclient/etc/postfix/main.cf

@@ -54,3 +54,5 @@ enable_long_queue_ids = yes
 # Disable smtputf8 support since exchange does not support it
 smtputf8_enable = no
 
+# Allow configuring maximum message size
+message_size_limit = {{ pillar['message_size_limit'] }}