Starting with microversion 2.37, network object in servers.CreateOpts can be a string: auto or none. none allows to create a VM without a network.

Starting with microversion 2.37, this field is required and the special string values auto and none can be specified for networks. auto tells the Compute service to use a network that is available to the project, if one exists. If one does not exist, the Compute service will attempt to automatically allocate a network for the project (if possible). none tells the Compute service to not allocate a network for the instance. The auto and none values cannot be used with any other network values, including other network uuids, ports, fixed IPs or device tags. These are requested as strings for the networks value, not in a list. See the associated example.

(c) https://docs.openstack.org/api-ref/compute/?expanded=create-server-detail#create-server

refs:

• https://github.com/openstack/nova/blob/72224775fb6a2eda0ceec51dfcb971fc50d4ea02/nova/objects/network_request.py#L25..L26
• https://github.com/openstack/nova/blob/72224775fb6a2eda0ceec51dfcb971fc50d4ea02/nova/api/openstack/compute/servers.py#L398..L445