ensure zeros are written out when clearing volumes

ensure zeros are written out when clearing volumes

Note O_DIRECT is _not_ used when copying from /dev/zero
and there are issues with enabling that (see 444cd542).
Therefore we arrange to have dd issue an fdatasync()
to ensure the data is persisted, lest it be discarded
from the write cache when the device is unprovisioned.

* cinder/volume/drivers/lvm.py (_copy_volume): Add 'conv=fdatasync'
to the dd option list if O_DIRECT isn't used when clearing
(which it won't as descrived above).

Fixes bug: 1100363
Change-Id: I76789557754ebaeb6d52bb34548a2ef17808fbf6