test_s3: extend multi-delete test

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
test_s3: extend multipart test

Make test use a multipart upload with different part sizes, to exercise
code that triggered #7935.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
test_s3: add some debug prints for multi region test

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>