Debian: either split components or remove non-free and contrib all together

[dirk-thomas]

In order for ros-infrastructure/ros_buildfarm#530 to work the script needs to:

• either generate separate deb lines for main, non-free, and contrib or
• remove non-free and contrib (aligning what other Debian images use)

[tfoote]

If reformatting this will make our script work that's good. Though we can't always rely on upstream never doing this. We should probably discuss removing them in #4