requirements.txt: work around pip 1.4 issue

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

related to issue #5261

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
s3_test: add test for listing objects with special prefix

Check for ceph issue #5362. Prefix starts with underscore.

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