Use Downward API to inject namespace

[danielezonca]

It is possible to use Downward API to inject namespace metadata and this is in general a good practice for every deployment / pod so that it has in the env similar informations.
This also removes the codepath that verify if namespace is retrieved

See
https://kubernetes.io/docs/concepts/workloads/pods/downward-api/#downwardapi-fieldRef
https://kubernetes.io/docs/tasks/inject-data-application/environment-variable-expose-pod-information/