We use the zuul_scheduler_start flag to determine if we want to start
the zuul-scheduler when new containers show up. Unfortunately we weren't
setting zuul_scheduler_start in prod so we failed with this error:
error while evaluating conditional (zuul_scheduler_start | bool): 'zuul_scheduler_start' is undefined
Fix this by treating an unset var as equivalent to a set truthy var
value. We do this instead of always setting the var to false in prod as
it simplifies testing.
Change-Id: I1f1a86e80199601646c7f2dec2a91c5d65d77231
5acf91a3f6