Update status for CVE-2019-15753/python-os-vif

The issue got introduce with upstream commit 1f6fed6 ("remove use of
brctl from vif_plug_linux_bridge") which first entered os-vif in 1.15.0.

Additional information in https://bugs.launchpad.net/os-vif/+bug/1837252/comments/20

As such buster and stretch are both not affected by the issue.