Publish cloud.instance.id ecs field with k8s nodes

[MichaelKatsoulis]

Closes #144

[tommyers-elastic]

nits:

do we need the empty parents list at all?

we can make this more DRY as follows:

options := []internal.AssetOption{
	internal.WithAssetTypeAndID(assetType, assetId),
	internal.WithNodeData(o.Name, assetProviderId, &assetStartTime),
	internal.WithIndex(assetType, indexNamespace),
}

if instanceId != "" {
	options = append(options, internal.WithCloudInstanceId(instanceId))
}

internal.Publish(publisher, options...)

[MichaelKatsoulis]

Nice. Thanks! Regarding the parents of the nodes I guess it will be always empty list. So we don't need to publish an empty list. Or is it a field that should be always there regardless if it is empty or not? That I don't know