Filter only kops ec2 resource with type ec2.ResourceTypeSubnet (#1752)

Ensure that only resource with type ec2.ResourceTypeSubnet are processed by ListSubnets 


When subnets carries AssociatedElasticIp or AssociatedNatgateway tag,
ListSubnets function doesn't always return resources with
ec2.ResourceTypeSubnet type

These resources doesn’t have Obj property causing type conversion from
nil into *ec2.Subnet to panic

Issue #1410

pkg/kops/kops.go

@@ -54,6 +54,9 @@ func (k *Wrapper) UseVPC(provider api.ClusterProvider, spec *api.ClusterConfig)
 	}
 
 	for _, subnet := range allSubnets {
+		if subnet.Type != ec2.ResourceTypeSubnet {
+			continue
+		}
 		subnet := subnet.Obj.(*ec2.Subnet)
 		for _, tag := range subnet.Tags {
 			if k.isOwned(tag) {