Use S3 ninja project for integration tests of S3-based staging area

[fschopp]

Integration tests for the S3-based staging area implementation currently assume that appropriate environment variables are set that point to an S3 bucket of a real AWS account. It is preferable to instead mock S3 for tests, so that testing neither incurs costs nor requires sharing AWS credentials.

The S3 Ninja project may provide such a mock implementation suitable for our needs.