This moves off use of the purged s3a://landsat-pds bucket, so fixing tests
which had started failing.
* Adds a new class, PublicDatasetTestUtils to manage the use of public datasets.
* The new test bucket s3a://usgs-landsat/ is requester pays, so depends upon
HADOOP-14661.
Consult the updated test documentation when running against other S3 stores.
Contributed by Daniel Carl Jones
6ab7b72cd6