Add WATCH_NAMESPACES env var to kubernetes operator

Provide the ability to set the namespaces watched by the operator using an env var. Whilst the additional config can still be used, the presence of the env var will take priority.

Reasons for issue

1. Operator will take effect of the setting immediately as pod will roll (rather than waiting for the config to be refreshed)
2. If the operator is to be olm bundled we will be able to set the target namespace using the following 

{{ env:}}

  {{ - name: WATCHED_NAMESPACE}}

      {{ valueFrom:}}

          {{ fieldRef:}}

             {{ fieldPath: metadata.annotations['olm.targetNamespaces']}}