Override osapi_volume_listen_port for test_workers

Override osapi_volume_listen_port for test_workers

After removing WSGI from Cinder, following test cases hit
"socket.error: [Errno 98] Address already in use" error.

- test_workers_set_default
- test_workers_set_good_user_setting
- test_workers_set_zero_user_setting

The root cause is that these test cases try to use 8776 port,
but the port is used by c-api if the service exists.
This patch override default port number to avoid the error.

Change-Id: I8e2b505c2aff511969cd55851272b10c013aef6c
Closes-Bug: #1527566