Foxy upgrade

[stonier]

• [x] Remove any mentions of Chooser → https://github.com/splintered-reality/py_trees_ros/pull/166
• [x] Updated deprecated Node's set_parameter_callback → https://github.com/splintered-reality/py_trees_ros/pull/166
• [x] Launch is deprecating node_name → name, node_executable → executable → https://github.com/splintered-reality/py_trees_ros_tutorials/pull/43
• [x] Launch does not seem to respect output=screen and emulate_tty any more
  • All log levels goes to one stream now. stderr (see the Foxy Changelog) which goes to screen regardless of whether output is set to log or screen. To change what log level appears, you need to pass arguments=['--ros-args', '--log-level', 'WARN'], when creating the node.
• [x] Version bump to 2.1 and release
• [x] Read the docs builds on py_trees_ros & py_trees_ros_tutorials 2.1.x branches
• [x] Update readme release matrix
• [ ] Resolve watcher problems on Fast-RPS (#165)

[stonier]

Closing the issue out, the last item will be tracked by #165.