Dont list buckets in s3 tests (#318)

* Don't attempt to list all buckets in s3 tests

The tests used a ListBuckets operation which requires privileges that
I'm not comfortable giving to Travis, and which aren't actually
necessary.  We know what the bucket name is so we don't need to scan
each and every bucket, we can just open the one we want.

Also removed an if/then on test bucket creation which caused race
conditions.

* Reduce create/delete bucket churn in s3 tests

The s3 tests were deleting and creating the same bucket over and over
which isn't necessary - we now create the bucket once before the tests
run and delete it after they're done.  This is a little faster and
appears to reduce the number of race conditions that we hit.

* Respond to code review comments

#318 (review)

 - No more global "s3" handle - allocate one locally when needed

 - Document setUpModule and tearDownModule

 - Document when we create/delete the test bucket

 - Put populate_bucket back where it was

 - Call cleanup_bucket after each test