vhost_max: add a Cuttlefish validator

Author: michaelklishin

deps/rabbit/priv/schema/rabbit.schema

@@ -898,7 +898,7 @@ end}.
 }.
 
 {mapping, "vhost_max", "rabbit.vhost_max",
-  [{datatype, [{atom, infinity}, integer]}]}.
+  [{datatype, [{atom, infinity}, integer]}, {validators, ["non_negative_integer"]}]}.
 
 {translation, "rabbit.vhost_max",
  fun(Conf) ->