Document neutron agent cleanup

In some cases Neutron agents that was required in OSP 17 are no longer
needed. For example in OSP 17 the Neutron DHCP agent was required to
support Ironic provisioning, in RHOSO 18 OVN can provide the DHCP
service for Ironic provisioning.

This change adds some explanation, and an example showing how to
identify and delete neutron agents that has been permanently retired
post adoption.

Related PR: #937
Related Jira: OSPRH-15749

Author: hjensas

docs_user/modules/proc_adopting-networker-services-to-the-data-plane.adoc

@@ -342,6 +342,33 @@ EOF
 [NOTE]
 Alternatively, you can include the Networker node set in the `nodeSets` list before you deploy the main `OpenStackDataPlaneDeployment` CR. You cannot add new node sets to the `OpenStackDataPlaneDeployment` CR after deployment.
 
+. Clean up any {networking_first_ref} agents that are no longer running.
++
+[NOTE]
+In some cases, agents from the old data plane that are replaced or retired remain in {rhos_acro}. The function these agents provided might be provided by a new agent that is running in {rhos_acro}, or the function might be replaced by other components. For example, DHCP agents might no longer be needed, since OVN DHCP in {rhos_acro} can provide this function.
+
+.. List the agents.
++
+----
+$ oc exec openstackclient -- openstack network agent list
++--------------------------------------+------------------------------+--------------------------+-------------------+-------+-------+----------------------------+
+| ID                                   | Agent Type                   | Host                     | Availability Zone | Alive | State | Binary                     |
++--------------------------------------+------------------------------+--------------------------+-------------------+-------+-------+----------------------------+
+| e5075ee0-9dd9-4f0a-a42a-6bbdf1a6111c | OVN Controller Gateway agent | controller-0.localdomain |                   | :-)   | UP    | ovn-controller             |
+| 856960f0-5530-46c7-a331-6eadcba362da | DHCP agent                   | controller-1.localdomain | nova              | XXX   | UP    | neutron-dhcp-agent         |
+| 8bd22720-789f-45b8-8d7d-006dee862bf9 | DHCP agent                   | controller-2.localdomain | nova              | XXX   | UP    | neutron-dhcp-agent         |
+| e584e00d-be4c-4e98-a11a-4ecd87d21be7 | DHCP agent                   | controller-0.localdomain | nova              | XXX   | UP    | neutron-dhcp-agent         |
++--------------------------------------+------------------------------+--------------------------+-------------------+-------+-------+----------------------------+
+----
+
+.. If any agent in the list shows `XXX` in the `Alive` field, verify the Host and Agent Type, if the functions of this agent is no longer required, and the agent has been permanently stopped on the {rhos_prev_long} host. Then, delete the agent by running:
++
+----
+$ oc exec openstackclient -- openstack network agent <agent_id>
+----
+* Replace `<agent_id>` with the ID of the agent to delete, for example, `856960f0-5530-46c7-a331-6eadcba362da`.
+
+
 .Verification
 
 . Confirm that all the Ansible EE pods reach a `Completed` status: