Use a DynamoDB table to coodinate state locking in S3.
We use a simple strategy here, defining a key containing the value of
the bucket/key of the state file as the lock. If the keys exists, the
locks fails.
TODO: decide if locks should automatically be expired, or require manual
intervention.
This change allows the user to specify `-backend-config="encrypt=1"`
to tell S3 to encrypt the data that's in the bucket when using S3
for remote config storage.
The encryption uses "Amazon S3-managed encryption keys" so it should
not require any further user intervention.
A line was added to the unit test just for coverage.
The acceptance test was modified to:
a) Use encryption
b) Push some test data up to the bucket created to ensure
that Amazon accepts the header.
James Bardin