ITestDynamoDBMetadataStore can leak (large) DDB tables in test failures/timeout

This is me doing some backporting of patches from branch-3.2, so it may be an intermediate condition but

1. I'd noticed I wasn't actually running ITestDynamoDBMetadataStore
2. so I set it up to work with teh right config opts (table and region)
3. but the tests were timing out
4. looking at DDB tables in the AWS console showed a number of DDB tables "testProvisionTable", "testProvisionTable", created, each with "500 read, 100 write capacity (i.e. ~$50/month)

I haven't replicated this in trunk/branch-3.2 itself, but its clearly dangerous. At the very least, we should have a size of 1 R/W in all creations, so the cost of a test failure is neglible, and then we should document the risk and best practise.

Also: use "s3guard" as the table prefix to make clear its origin