ceph-disk: always check zap is applied on a full device

ceph-disk: always check zap is applied on a full device

zap only makes sense on a full device and this is verified for

    ceph-disk prepare --zap-disk

but not for

    ceph-disk zap

Unify the two and add a test to check that it fails as expected when
trying to zap a directory.

http://tracker.ceph.com/issues/11272 Fixes: #11272

Signed-off-by: Loic Dachary <ldachary@redhat.com>
(cherry picked from commit 8e1bcf21f090fc98ac9b495d76614e3012f5872a)

Conflicts:
	src/test/ceph-disk.sh : trivial resolution