Support AWS job stores inside existing buckets

[unito-bot]

AGC really would like us to work in one bucket, and without SimpleDB there's no good reason we can't work in a prefix of a shared bucket, instead of having one entire bucket per AWS-based workflow run.

┆Issue is synchronized with this Jira Story ┆Issue Number: TOIL-1115

[larryns]

This change would be really helpful.