Fix issue attribute error in queue
Fix issue attribute error in queue

When multinode jobs are queued, the master scheduler
fails to start single node jobs for idle devices because
of an exception in queue handling:
AttributeError: 'QuerySet' object has no attribute 'append'

Fix public devices check to ensure that devices remains a
list, not a QuerySet as we need to use append().

Change-Id: I52110ab3eebf7ee4af98fab7c2a2a88bd7e01b77

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,OSSystems/lava-server,Linaro/lava-server,OSSystems/lava-server,OSSystems/lava-server