Nitpicky changes

These are the changes we discussed today:
- condor_ce_generator was renamed to condor_ce_config_generator. I also found and changed text that referred to condor_ce_collector_generate -- I assume that was an earlier name?
- The execvp in the generator was replaced by subprocess.call and I checked the exit code (raising CalledProcessError on failure).
- The text in the auto-generated config file was reorganized and tweaked to be more obvious about where changes should be made.