MXS-2540 Add monitor support to new config system

The configuration system needs to be aware of standard monitor
parameters.

include/maxscale/config2.hh

@@ -41,7 +41,8 @@ class Specification
 public:
     enum Kind
     {
-        FILTER
+        FILTER,
+        MONITOR
     };
 
     using ParamsByName = std::map<std::string, Param*>;