packaging: setup: drop tables on restore

Without this change, running 'engine-backup.sh --mode=restore' twice
causes some lines to be duplicated in the database.

Bug-Url: https://bugzilla.redhat.com/1020277
Change-Id: I8bdd8c01349b3cb2a4b1ab79a048c09479c23f27
Signed-off-by: Yedidyah Bar David <didi@redhat.com>