When non-DRBD disks are used for an instance,
“lu.needed_locks[locking.LEVEL_NODE]” is set to “locking.ALL_SET” (which
is None). The assertion will then fail as None evaluates to False.

Reported by Constantinos Venetsanopoulos.

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