Update ctrl_bps_panda/config/bps_usdf.yaml to allow for local custom setup.

I merged the PR, so the hook will be available in next weekly.

Thanks Michelle. For this ticket, I'll add the doc/changes file, and also move "pwd; ls -al;" to runnerCommand as you suggested.

About change bps_idf.yaml and bps_panda.yaml, we should do it in a seperate ticket? I'll check more on those.

Merge approved (after adding doc/changes file)

• Needs doc/changes file.

• Is there documentation somewhere (panda.lsst.io maybe) where custom_lsst_setup needs to be mentioned?

• I think this meets the user's request. However, in this or some other ticket, similar changes should be made to the bps_idf.yaml as this feature is not usdf specific (even though the value of custom_lsst_setup is different). Also, after pulling this out, can runnerCommand now only appear in the central bps_panda.yaml? (maybe another "extra_docker_env" bps setting needs to be introduced and only have a value at IDF?).

I added the hook as Eric requested. From my test it works. However I'm not quite sure this is what we should/want to have in the bps_usdf.yaml in general. 

@Michelle Gower Can you help review this ticket? I remember you have some comments at last week's CM meeting about this.