bdev: allow ignoring of size in Assemble()

This patch changes the DRBD8 class (the only one to use the size in
Assemble) to ignore the size in Assemble when a zero size is passed.
This will allow activation of disks even when the size recorded in the
configuration is wrong.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>