LUClusterVerify: Ignore /proc/drbd if DRBD is disabled

This fixes issue 190. The problem was that the check for DRBD was
enabled if LVM storage is used and didn't depend at all on whether DRBD
is enabled.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
(cherry picked from commit 3d8ae327)