Comment 0 for bug 1940312

* High level description: When we get a list of segments to choose from, and the first segment is already allocated, it fails right away returning RetryRequest exception, the the other segments are never tried out.

I explain it a little further on the comments of PatchSet 1 here: https://review.opendev.org/c/openstack/neutron/+/803986/1

This actually works at master due to a side effect of a refactoring that was done on https://opendev.org/openstack/neutron/commit/6eaa6d83d7c7f07fd4bf04879c91582de504eff4 to randomize the selection of segments, but on stable/ussuri, when not specifying the provider_network_type, we got into a situation where we had segments to allocate in vlan but neutron was allocating vxlan instead.

* Pre-conditions: network-segments plugin enabled and several vlan project networks created on the system

* Step-by-step reproduction steps: openstack --os-username 'project1_admin' --os-password '******' --os-project-name project1 --os-auth-url http://keystone.openstack.svc.cluster.local/v3 --os-user-domain-name Default --os-project-domain-name Default --os-identity-api-version 3 --os-interface internal --os-region-name RegionOne network create network11

* Expected output: network created successfully (there was available ranges)

* Actual output: HttpException: 503, Unable to create the network. No tenant network is available for allocation.

* Version:
  ** OpenStack version: stable/ussuri
  ** Linux distro: Centos 7
  ** Deployment: StarlingX Openstack

* Perceived severity: Major - System is usable in some circumstances