Fix plugin loading order to match docs

[thettasch-samxl]

Fixes #130.

The documentation states that "registerNodesIntoFactory is a callback invoked after the plugins were registered into the BT::BehaviorTreeFactory. ". This PR updates the code to match the described order.

See:

BehaviorTree.ROS2/behaviortree_ros2/include/behaviortree_ros2/tree_execution_server.hpp

Lines 104 to 112 in 6c6aa07

	/**
	* @brief registerNodesIntoFactory is a callback invoked after the
	* plugins were registered into the BT::BehaviorTreeFactory.
	* It can be used to register additional custom nodes manually.
	*
	* @param factory The factory to use to register nodes
	*/
	virtual void registerNodesIntoFactory(BT::BehaviorTreeFactory& factory)
	{}

and the current call order, which does not match the above description:

BehaviorTree.ROS2/behaviortree_ros2/src/tree_execution_server.cpp

Lines 116 to 119 in 6c6aa07

	// user defined method
	registerNodesIntoFactory(p_->factory);
	// load plugins from multiple directories
	RegisterPlugins(p_->params, p_->factory, node_);