Ignore "unspecified" externalIP during LB create

Previously we always passed `Address: externalIP.String()` while
creating a loadbalancer VIP.  This passed "0.0.0.0" when externalIP was
unspecified, effectively making it mandatory to specify an externalIP.

This change correctly leaves `Address` unspecified when externalIP is
unspecified (has a zero value).

(Thanks to @justinsb for the report)

pkg/cloudprovider/openstack/openstack.go

@@ -607,15 +607,19 @@ func (lb *LoadBalancer) CreateTCPLoadBalancer(name, region string, externalIP ne
 		}
 	}
 
-	vip, err := vips.Create(lb.network, vips.CreateOpts{
+	createOpts := vips.CreateOpts{
 		Name:         name,
 		Description:  fmt.Sprintf("Kubernetes external service %s", name),
-		Address:      externalIP.String(),
 		Protocol:     "TCP",
 		ProtocolPort: ports[0].Port, //TODO: need to handle multi-port
 		PoolID:       pool.ID,
 		Persistence:  persistence,
-	}).Extract()
+	}
+	if !externalIP.IsUnspecified() {
+		createOpts.Address = externalIP.String()
+	}
+
+	vip, err := vips.Create(lb.network, createOpts).Extract()
 	if err != nil {
 		if mon != nil {
 			monitors.Delete(lb.network, mon.ID)