Bug 1849166

Description of problem: After performing a compute scale down on RHOS 16.1, network agents for the deleted compute host still exist and for OVN deployments these agents cannot be deleted. For OVS deployments, the agents reappear after they have been deleted.

Related Docs BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1841011

How reproducible: always

Steps to Reproduce:
1. Deploy RHOS 16.1 with at least 2 compute nodes
2. Scale down 1 compute host
3. OVN agents for the deleted compute are still up and running

Actual results:
(overcloud) [stack@undercloud-0 ~]$ openstack network agent list
+--------------------------------------+----------------------+---------------------------+-------------------+-------+-------+-------------------------------+
| ID                                   | Agent Type           | Host                      | Availability Zone | Alive | State | Binary                        |
+--------------------------------------+----------------------+---------------------------+-------------------+-------+-------+-------------------------------+
| 79362159-1532-4473-b4cf-295ac7970cb9 | OVN Controller agent | compute-0.redhat.local    | n/a               | XXX   | UP    | ovn-controller                |
| 49991374-00fa-4d70-9dc1-598e9c4c83d9 | OVN Metadata agent   | compute-0.redhat.local    | n/a               | XXX   | UP    | networking-ovn-metadata-agent |
| a0eeb079-4fe1-4bb0-be14-0a57a0c487ce | OVN Controller agent | compute-1.redhat.local    | n/a               | :-)   | UP    | ovn-controller                |
| 2cee6616-96c5-4bd8-89e9-12416e663a3d | OVN Metadata agent   | compute-1.redhat.local    | n/a               | :-)   | UP    | networking-ovn-metadata-agent |
| 8b8b2933-9ab1-4740-9358-fcabf5b4b88e | OVN Controller agent | controller-0.redhat.local | n/a               | :-)   | UP    | ovn-controller                |
+--------------------------------------+----------------------+---------------------------+-------------------+-------+-------+-------------------------------+
(overcloud) [stack@undercloud-0 ~]$  openstack network agent delete 79362159-1532-4473-b4cf-295ac7970cb9
Failed to delete network agent with ID '79362159-1532-4473-b4cf-295ac7970cb9': BadRequestException: 400: Client Error for url: http://10.0.0.148:9696/v2.0/agents/79362159-1532-4473-b4cf-295ac7970cb9, Bad agent request: OVN agents cannot be deleted.
1 of 1 network agents failed to delete.
(overcloud) [stack@undercloud-0 ~]$  openstack network agent delete 49991374-00fa-4d70-9dc1-598e9c4c83d9
Failed to delete network agent with ID '49991374-00fa-4d70-9dc1-598e9c4c83d9': BadRequestException: 400: Client Error for url: http://10.0.0.148:9696/v2.0/agents/49991374-00fa-4d70-9dc1-598e9c4c83d9, Bad agent request: OVN agents cannot be deleted.
1 of 1 network agents failed to delete.

Expected results:
Agents for the deleted node should not exist at all and should be done as a part of tripleo scale down tasks 

Additional info:
Documentation: Section 15.3. Removing Compute nodes
https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.0/html/director_installation_and_usage/scaling-overcloud-nodes

From the related BZ, notes:
```
Dan Macpherson tested this on 16.1 - results:
9. Remove the Open vSwitch agent from the node: 

DDF: agents cant be deleted

Dan: This is true for OVN agents. This step can be deleted.

For OVS agents (not applicable to this procedure tho): You can delete the agent but it gets recreated after you delete it. So we probably need to find out what keeps recreating the agent and disable it if possible.
```