[PM] PM Failed to delete port when concurrent

[yanmo96]

Test case: alcor-create-and-delete-ports

• create and delete 10 ports per network, do it 2 times with 2 concurrency.

Issue:

• During clean up it will show there are still ports in the subnet, so subnet can not be deleted.

Root cause:

• Two process are accessing the cache at same time, and one process put back the port already been by the other process.

2021-08-25 11:01:40.827 26006 DEBUG neutronclient.v2_0.client [-] Error message: {"timestamp":"2021-08-25T18:01:40.826+0000","status":409,"error":"Conflict","message":"There is some ports in the subnet, we can Not delete subnet","path":"/project/05fd124b97484cb8963a58a8f1b6a268/subnets/ee9e889e-9be4-437f-95c9-ccf225b6e87c"} _handle_fault_response /root/rally_venv/lib/python3.6/site-packages/neutronclient/v2_0/client.py:262

[pkommoju]

Submitted PR# 683.

https://github.com/futurewei-cloud/alcor/pull/683