Assemble DRBD using the known size

This patch changes DRBD disk attachment to force the wanted size, as opposed to
letting the device auto-discover its size.

This should make the disks more resilient with regard to small differences in
size (e.g. due to LVM rounding). This still works with regard to disk
growth, but the instances needs to be fully restarted (including disks)
in that case.

This passes a full burning without problems, but it's still a tricky
change - if the config.data is not synced with the reality, we might
tell DRBD a wrong size. At least this will fail outright (and not
introduce silent errors), as DRBD (per a quick check at the sources)
tracks the size in the meta-dev and also does not allow shrinking
consistent devices.

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