Volume clearing is intended to provide safety against
data leaking between different volumes/tenants when
backing storage is reused. dd is sufficient to
accomplish this, and since 'shred' does not add anything
meaningful and has not seen much real-world use,
we should remove it for simplicity.
This change makes 'volume_clear=shred' call dd and
warns that this option will be removed.
Change-Id: I190dc22c5edb6efc56b98c5eac870a35c03fcd3f
c8a5e7d8ce