Ensure database.max_overflow == DEFAULT.api_workers (num cpus).

This causes bugs in Neutron if not set like this, under heavy parallel
load, but the root cause is unknown on the bug tracker.
(api_workers defaults to the number of CPUs if unset.)
1 job for master in 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #769
testing

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