Remove superflous out-of-date comment

The decision since that time that unification of configuration
for each backend will not be attempted.
Disable bucket name validation in s3_worker

This can cause problems when one is using IAM policies that
restrict actions to a prefix in the bucket.

Also avoids one API call in the common case where the bucket is valid,
although later calls that touch the bucket for the first time must be
ready for an invalid bucket to raise exceptions.
Alphabetize imports
Rework credential configuration management

Credentials are now opaque objects except to the specific blob store
implementation that uses them.  This is to allow worker function
signatures to retain the same signature between backend storage
implementations even though different backends have different methods
of authentication that go beyond the account name/password approach.

While doing this, teach the S3 credential regime a 'security token'
parameter, which allows use of keys vended by AWS STS.
Add tests stressing AWS STS credentials and functionality
Remove dead parameter for .delete_all
Fix thinko: creds has no prefix attribute, layout does