Use the aws provider code to create the clients for the s3 backend, so
that all the behavior matches that of the provider.
Remove the fake creds from the test, as the aws provider will attempt to
validate them.
Move the S3 State from a legacy remote state to an official backend.
This increases test coverage, uses a set schema for configuration, and
will allow new backend features to be implemented for the S3 state, e.g.
"environments".
James Bardin