Fix rosdep install for non-ROS base images

[jpbusch]

Always installing ros-${ROS_DISTRO}-ros-core and sourcing ROS before rosdep ensures that ROS_VERSION is set for every base-image (especially for using non-ROS base-images) and that standard ROS2 CLI tools like rosrun are always installed.

[lreiher]

Changes as dicussed:

• introduce ROS_VERSION as build argument and environment variable
• set ROS_VERSION automatically based on ROS_DISTRO
  • default to 2
  • only automatically set to 1 if ROS_DISTRO == noetic
  • no explicit list of ROS2 distros required in this case
• the above is not necessary; we can simply check for noetic, else assume ROS_VERSION=2
• always install ros-core to guarantee availability of ros2 CLI and more