Set concurrency default value
Set concurrency default value

This commit sets the default value 0 for tempest run command.
Because stestr's run_command() requires an int value for that, but we
passes `None` if users don't specify a concurrency value. We found that
behavior with recent stestr change[0]. But we also have the bug in
tempest side.

[0] https://github.com/mtreinish/stestr/issues/231

Change-Id: I83f76acb6a106f7401904a910a5dced47c2db839

openstack/tempest,masayukig/tempest,cisco-openstack/tempest,openstack/tempest,masayukig/tempest,cisco-openstack/tempest