MXS-2574: Add PATCH for /users/inet endpoint

The alteration of user passwords is now done inside MaxScale. This
prevents the possibility of a user locking themselves out.

server/core/internal/config_runtime.hh

@@ -386,6 +386,17 @@ bool runtime_create_user_from_json(json_t* json);
  */
 bool runtime_remove_user(const char* id, enum user_type type);
 
+/**
+ * @brief Alter admin user password
+ *
+ * @param user Username
+ * @param type Type of the user
+ * @param json JSON defining the new user
+ *
+ * @return True if the user was altered successfully
+ */
+bool runtime_alter_user(const std::string& user, const std::string& type, json_t* json);
+
 /**
  * @brief Alter core MaxScale parameters from JSON
  *