Add a crude disable for DRBD barriers

Ideally we want to/will have per-device DRBD controls of disk/metadata
flushes. In the meantime, we want at least a disable of the barrier
functionality for cases where one has battery-backed caches.

Background: DRBD has four mechanism of handling ordered disk-writes.
From the drbdsetup man-page, these are: barrier, flush, drain and none.
DRBD prior to 8.2 only has drain and none. This patch makes all 8.x
versions of DRBD disable all methods, and revert to none, in case one
fully trusts batteries (either UPS for the whole system or battery for
NVRAM).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>