We can't really detect all the authentication data in a sane manner for
disk backing chains. Since the old RBD parser parses it in some cases as
the argv->XML convertor requires it, we can't just drop it.

Instead clear any detected authentication data in the code paths related
to disk backing chain lookup and fix the tests to cope with the change.

https://bugzilla.redhat.com/show_bug.cgi?id=1544659Signed-off-by: NPeter Krempa <pkrempa@redhat.com>