[Bug 1890432] Fix included in openstack/neutron 18.0.0.0rc1

[OpenStack Infra]

This issue was fixed in the openstack/neutron 18.0.0.0rc1 release
candidate.

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https://bugs.launchpad.net/bugs/1890432

Title:
  Create subnet is failing under high load with OVN

Status in neutron:
  Fix Committed
Status in neutron package in Ubuntu:
  Fix Released
Status in neutron source package in Focal:
  Triaged
Status in neutron source package in Groovy:
  Triaged

Bug description:
  Under a high concurrency level create subnet is starting to fail.
  (12-14% failure rate) The bundle is OVN / Ussuri.

  neutronclient.common.exceptions.Conflict: Unable to complete operation
  on subnet  This subnet is being modified by another concurrent
  operation.

  Stacktrace: https://pastebin.ubuntu.com/p/sQ5CqD6NyS/
  Rally task:

  {% set flavor_name = flavor_name or "m1.medium" %}
  {% set image_name = image_name or "bionic-kvm" %}

  ---
    NeutronNetworks.create_and_delete_subnets:
      -
        args:
          network_create_args: {}
          subnet_create_args: {}
          subnet_cidr_start: "1.1.0.0/30"
          subnets_per_network: 2
        runner:
          type: "constant"
          times: 100
          concurrency: 10
        context:
          network: {}
          users:
            tenants: 30
            users_per_tenant: 1
          quotas:
            neutron:
              network: -1
              subnet: -1

  Concurrency level set to 1 instead of 10 is not triggering the issue.

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