Currently Magnum fails to delete certificates when barbican
cert manager is used. The code was copied from neutron-lbaas and
they have different usecase. In our case, certificate is managed by
Magnum not users, so we should delete certificates when deleting bay.
So this patch deletes all the certs related to a bay.
Change-Id: I5aab01641b9447153911680c5f68e5fe2c5a1409
Closes-bug: #1587033