Fix `drbdsetup show` parsing for DRBD 8.4

In DRBD 8.4, Ganeti does no longer use indexed external meta data but
flexible external metadata. Therefore, there is no longer a meta data
index in the output of `drbdsetup show`.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>