Consider allowing passing existing ROS2 Node object to `setup()` method of `py_trees_ros.trees.BehaviourTree`

[sea-bass]

In py_trees_ros.trees.BehaviourTree, the setup() method always creates a new node named "tree".

I have two requests, which if you agree with, I would be happy to put up a PR:

1. Can we consider optionally passing in an existing ROS2 node object in setup(), instead of having the tree always create its own node? This allows us to instantiate a ROS enabled BT inside an already created Node (or Node-derived) object.
2. Right now, the node name is defaulted to "tree". Consider adding an optional argument to change the name, provided an existing node isn't passed in as per the previous item.