MXS-2219 Dynamically create Clustrix servers

MaxScale server objects are now created for all Clustrix nodes.
Currently the name is "Clustrix-Server-N" where N is the number
of the node.

The server is created using runtime_create_server() that has been
modified so that it optionally will not persist the created server.
That is probably just a temporary solution as a monitor should not
need to include .../core/internal-stuff.

server/core/internal/config_runtime.hh

@@ -57,7 +57,8 @@ bool runtime_create_server(const char* name,
                            const char* address,
                            const char* port,
                            const char* protocol,
-                           const char* authenticator);
+                           const char* authenticator,
+                           bool        serialize = true);
 
 /**
  * @brief Destroy a server