tested in 4.8 - primarySubnet attribute is mandatory.

twoNICs_machineset.yaml

apiVersion: machine.openshift.io/v1beta1
kind: MachineSet
metadata:
  name: ostest-dzghr-data-0
  namespace: openshift-machine-api
spec:
  replicas: 1
  selector:
    matchLabels:
      machine.openshift.io/cluster-api-cluster: ostest-dzghr
      machine.openshift.io/cluster-api-machineset: ostest-dzghr-data-0
  template:
    metadata:
      labels:
        machine.openshift.io/cluster-api-cluster: ostest-dzghr
        machine.openshift.io/cluster-api-machine-role: worker
        machine.openshift.io/cluster-api-machine-type: worker
        machine.openshift.io/cluster-api-machineset: ostest-dzghr-data-0
    spec:
      metadata: {}
      providerSpec:
        value:
          apiVersion: openstackproviderconfig.openshift.io/v1alpha1
          cloudName: openstack
          cloudsSecret:
            name: openstack-cloud-credentials
            namespace: openshift-machine-api
          flavor: m4.xlarge
          image: ostest-dzghr-rhcos
          kind: OpenstackProviderSpec
          metadata:
            creationTimestamp: null
          networks:
          - filter: {}
            subnets:
            - filter: {}
              uuid: 3059f555-271a-4e50-89e4-54dc542a8b7d
            - filter:
                name: data-subnet
          primarySubnet: 3059f555-271a-4e50-89e4-54dc542a8b7d
          securityGroups:
          - filter: {}
            name: ostest-dzghr-worker
          serverMetadata:
            Name: ostest-dzghr-worker
            openshiftClusterID: ostest-dzghr
          tags:
          - openshiftClusterID=ostest-dzghr
          trunk: true
          userDataSecret:
            name: worker-user-data