Add mountpoint config to zone configs

etc/emqx.conf

@@ -665,6 +665,15 @@ zone.external.max_mqueue_len = 1000
 ## Value: false | true
 zone.external.mqueue_store_qos0 = true
 
+## All the topics will be prefixed with the mountpoint path if this option is enabled.
+##
+## Variables in mountpoint path:
+##  - %c: clientid
+##  - %u: username
+##
+## Value: String
+## zone.external.mountpoint = devicebound/
+
 ##--------------------------------------------------------------------
 ## Internal Zone
 
@@ -715,6 +724,15 @@ zone.internal.max_mqueue_len = 1000
 ## Value: false | true
 zone.internal.mqueue_store_qos0 = true
 
+## All the topics will be prefixed with the mountpoint path if this option is enabled.
+##
+## Variables in mountpoint path:
+##  - %c: clientid
+##  - %u: username
+##
+## Value: String
+## zone.internal.mountpoint = cloudbound/
+
 ##--------------------------------------------------------------------
 ## Listeners
 ##--------------------------------------------------------------------

priv/emqx.schema

@@ -842,6 +842,10 @@ end}.
   {datatype, string}
 ]}.
 
+{mapping, "zone.$name.mountpoint", "emqx.zones", [
+  {datatype, string}
+]}.
+
 {translation, "emqx.zones", fun(Conf) ->
   Mapping = fun("retain_available", Val) ->
                     {mqtt_retain_available, Val};