Make --concurrency not required in account_generator cmd
Make --concurrency not required in account_generator cmd

--concurrency arg in account_generator cmd is required
with default value 1.

This should be non required and work with default value if not given.

Change-Id: I105bd4833e5a0da58d900493a1782943c5506ffa

vedujoshi/tempest,Juniper/tempest,sebrandon1/tempest,Tesora/tesora-tempest,cisco-openstack/tempest,Tesora/tesora-tempest,sebrandon1/tempest,vedujoshi/tempest,openstack/tempest,Juniper/tempest,masayukig/tempest,openstack/tempest,cisco-openstack/tempest,masayukig/tempest