Add --no-wait-for-sync when converting to drbd

Currently, when converting an instance from plain to DRBD, the
instance is blocked during the entire resync period. This patch adds
the --no-wait-for-sync so that the operation finishes as soon as the
DRBD sync has started, without waiting for the entire sync. This makes
the instance available much faster.

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