DRBD 8.4.3 does not support specifying volume ids directly while
creating the meta device. It requires a slightly different syntax (using
flex-external as volume id) which is fortunately also supported by (at
least) DRBD 8.4.2.

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