Move sanitizing Helm charts functionality to post-renderer

[invidian]

#727 adds use of Helm's post-renderer functionality. As we currently do some filtering on rendered manifests, like removing all Namespace objects (https://github.com/kinvolk/lokomotive/blob/master/pkg/components/util/helm.go#L175), we could perhaps move that into the post-renderer functionality. This will allow avoiding rendering helm charts twice, which causes some troubles.