Process pool initialization refinement.

If vti address is not given, pass None as vti_client to Console.__init__
Init pab for each process in the pool, or every process in the pool
share the same tmp directory and may have some kind of race condition on
the fetched files.

Test: > device --lease=True
Bug: 72474046
Change-Id: Ifeb08517615eb0bfd9b33ea5a25ac51911adc677