Non standard directory placeholder

[sverhoeven]

The adaptor on createDirectory("something") creates a blob in S3 with key something/___not__empty___.

When you create a folderin the S3 console, it creates an object with the above name appended by suffix "/" and that object is displayed as a folder in the S3 console. In minio a key ending in / is also treated as an directory.

The adaptor should treat keys ending with / as directories.

[sverhoeven]

Started in https://github.com/xenon-middleware/xenon-adaptors-cloud/tree/no-dir-placeholder branch, but half the integration tests fail so backed off for now