Switch hacking and pbr to merge-if-necessary.

They (probably accidentally) were set up with the cherry-pick merge
strategy, but we should confine that to openstack-infra for the time
being until we are sure we want to support it more widely.

Change-Id: Ic813fd8d5b320ada1d83b468d41a43d430d49a04
Reviewed-on: https://review.openstack.org/30955
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: David Ripton <dripton@redhat.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins