Make LoadBalancerStrategy Optional

[danehans]

Please describe the problem you have Currently, "LoadBalancerStrategy" is being defaulted for Envoy network publishing. This is fine when using the "LoadBalancerService" type (default), but does not make sense when using other network publishing types. For example (NodePortService):

spec:
  namespace:
    name: projectcontour
    removeOnDeletion: false
  networkPublishing:
    envoy:
      containerPorts:
      - name: http
        portNumber: 8080
      - name: https
        portNumber: 8443
      loadBalancer:
        providerParameters:
          type: AWS
        scope: External
      type: NodePortService
  replicas: 2

[danehans]

Requires https://github.com/projectcontour/contour-operator/issues/386