This adds named state (environment) support to the S3 backend.
A state NAME will prepend the configured s3 key with `env:/NAME/`.
The default state will remain rooted in the bucket for backwards
compatibility.
Locks in DynamoDB use the S3 key as the as the primary key value, so
locking will work as expected for multiple states.
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".