refactor(helm): allow using helm built-in namespace

[yonatankahana]

The change is fully backward compatible and allows the user to decide whether he uses the namespace from the values.yaml or the built-in namespace of helm (--namespace).
The change is made possible thanks to the ability to exclude namespaces by name and not only by annotation as was the case until PR #79.

Now you can set the namespace to null and use the built-in namespace, i.e:

helm install k8tz k8tz/k8tz --namespace=myothernamespace --create-namespace --set namespace=

The namespace in the values can be still used with existing namespace by setting createNamespace to false, i.e:

helm install k8tz k8tz/k8tz --set namespace=existingnamespace --set createNamespace=false

And in the future if we want, we can always make breaking changes by removing namespace completely from values.

Fix #91