Change joint_states subscription to SensorDataQoS

[adeschamps]

This changes the subscription from reliable to best_effort. This is helpful when visualizing a robot that's running on a wifi-connected device, and eliminates a warning when starting up drivers on a robot that publishes with best_effort.

[sloretz]

Newer versions of ROS 2 have an opt-in configurable QoS feature. Would you be willing to implement that here instead of changing the hard-coded QoS?

Design doc http://design.ros2.org/articles/qos_configurability.html
Example PR with the required changes: cra-ros-pkg/robot_localization#657

[sloretz]

Also, please target the ros2 branch for new development. If it works in ROS Rolling, then it can be considered for backporting into a stable distro like ROS Galactic.

[adeschamps]

Closing in favour of #179.

Thanks for the link to that design doc and the example PR -- both were very helpful.