MXS-1220: Add alter maxscale MaxAdmin command

A set of the core MaxScale parameters can now be altered at runtime. This
set consists of the authentication timeouts and the admin interface
authentication. Other parameters either can't be modified due to internal
limitations or aren't sensible to modify at runtime.

server/core/maxscale/config_runtime.h

@@ -132,6 +132,16 @@ bool runtime_alter_monitor(MXS_MONITOR *monitor, const char *key, const char *va
  */
 bool runtime_alter_service(SERVICE *service, const char* zKey, const char* zValue);
 
+/**
+ * @brief Alter MaxScale parameters
+ *
+ * @param name  Key to modify
+ * @param value New value
+ *
+ * @return True if @c key was one of the supported parameters
+ */
+bool runtime_alter_maxscale(const char* name, const char* value);
+
 /**
  * @brief Create a new listener for a service
  *