Remove support for rmw_connext_cpp from the scripts.

[clalancette]

The last distribution that supported this (Foxy) is now EOL.

Unfortunately, removing this is more complicated than I would have liked. In particular, the job templates were always setting '--ignore-rmw rmw_connext_dynamic_cpp', and then adding to that list if other RMWs should be ignored. But we no longer have an RMW that we always want to ignore, so we have to pass --ignore-rmw for each of the ones that we want to ignore (because there may not be any). To handle that, we had to change the argparse handling of --ignore-rmw to use the 'extend' action, and then we had to modify the entrypoint.sh to deal with the fact that there might be multiple --ignore-rmw flags, any of which could contain 'rmw_connextdds'.

Note that if we merge in https://github.com/ros2/ci/pull/713 first, this one will need to be rebased.

[clalancette]

All right, with #715 now merged and deployed, I've gone ahead and rebased this. This is now ready for another review.

[clalancette]

All right, I've gone ahead and deployed this change to the test_ jobs only. Here are some test jobs with this in place:

• Linux
• Linux-aarch64
• Windows
• Linux with ConnextDDS disabled

[clalancette]

All right, this is looking good. The normal jobs look OK, and the one where I specifically disabled connext did what I expected and didn't install it. I'm going to go ahead and merge and deploy this one, then move onto the next one.