Rename ROS master references to neutral form

[gonzalocasas]

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.