helm: missing permissions for Dask dashboard

[mdonadoni]

On DEV/QA/PROD, traefik is already deployed in the cluster, so we set traefik.enabled to false.

If Dask is enabled, reana-workflow-controllor creates a Middleware to expose the Dask dashboard. However, permissions to do so are missing when traefik.enabled is false:

reana/helm/reana/templates/roles.yaml

Lines 31 to 36 in d81a814

	# Traefik Middleware permissions
	{{- if and .Values.traefik.enabled .Values.dask.enabled }}
	- apiGroups: ["traefik.io"]
	resources: ["middlewares"]
	verbs: ["get", "list", "watch", "create", "update", "patch", "delete"]
	{{- end }}

This causes errors when trying to start any workflow using Dask.