Rename ROS master references to neutral form

Rename usage of `master` when referring to ROS to a neutral language alternative:
- when talking about `connection to ROS master`, reword to `connection to ROS` instead
- when we need to refer to the exact master node, use `roscore`
- ROS service names in docker compose files can be renamed to `roscore` for the coordination service
- the env variable that refers to MASTER will need to stay as it is because ROS needs it.