migrate_node.py bso num bug

[erkolson]

When --bso_num 0 is passed to the script, all 20 bsos are scanned/uploaded.

This is due to 0 evaluating to False here:
https://github.com/mozilla-services/syncstorage-rs/blob/master/tools/user_migration/migrate_node.py#L779-L780

I don't know that we need to have --start_bso and --end_bso as options anymore (making bso_num required), as in practice we will be splitting BSOs across multiple processes and not using a single process to move multiple BSOs. I'm not sure how it was able to migrated data from other BSOs given that it was only provided with a single bso file!

[erkolson]

I looked at the migration logs and found that it looped through 0 to 19 and uploaded the same 2 users that were in the provided bso file 20 times.