DRBD: ignore unreadable meta devices

The DRBD driver can ignore dead disks but not dead meta devices (for
which it refuses to configure the minor). To handle this case, we check
that the meta device is readable and if not we ignore it (the same as
when backend._RecursiveAssembleBD didn't find it).

A needed change is the move of some checks and initialisers before this
test (which is before the super().__init__ call), but that should be
fine.

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