Override security suite for Debian image builds
Override security suite for Debian image builds

Diskimage-builder defaults to a assuming the Debian security
archive's suite name will be something like stretch/updates or
buster/updates but ours are just stretch and buster without any
second subcomponent, so override this to an empty string instead.

Depends-On: https://review.opendev.org/688816
Change-Id: I34488e55c1b9bfe4c253c4d51346966fa69f1170

openstack-infra/project-config,openstack-infra/project-config