contrib: declare DRY_RUN empty
contrib: declare DRY_RUN empty

We must declare DRY_RUN but not assign anything, if we don't the script
will fail because of the -u option.

Fix /home/jenkins-build/build/workspace/ceph-container-build-ceph-base-push-imgs/ceph-container/contrib/ceph-build-config.sh: line 464: DRY_RUN: unbound variable

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

ceph/ceph-docker,ceph/ceph-docker