Support namespaces scoped watch

[wilfriedroset]

Hello

I plan to deploy kamaji on a multi-tenant k8s platform. I would like to restrict the namespaces watched by the operator. This is currently not possible at the moment. As such I propose to add a new flag --watch-namespaces along with the helm chart value watchNamespaces.
This way user can deploy their operator in their namespace without the risk to reconcile anything out of the scope.

Please note that I'm willing to pick up the related work as follow up of this feature request.