Remove skip_because in swift object expiry test

According to the comments in bug #1069849 container
listing will list an expired object (normal behavior).
This is up to the client to handle that a DELETE on
an expired object will return a 404 error.
delete_container method does not take care of any
HTTP return code for object deletion.

This patch remove the skip_because statement as the
test works as expected and not fails in tearDownClass.

This patch also add a test for metadata X-Delete-At.

Change-Id: I6c6efa151fd7b64684b25b049539680a3766e0cd
Merge "Remove skip_because in swift object expiry test"