helm: fix linting errors

[tiborsimko]

Current behaviour

helm lint currently reports an error for our chart:

$ helm lint helm/reana -f helm/configurations/values-dev.yaml
engine.go:189: [INFO] Fail:

 ERROR: You must have at least networking.k8s.io/v1beta1 to use ingressClass
==> Linting helm/reana

1 chart(s) linted, 0 chart(s) failed

Expected behaviour

Our charts should pass linting error-free.

Bonus

Introduce chart linting to the run-tests.sh and the CI process.

See also https://github.com/helm/chart-testing

[mdonadoni]

The error is coming from Traefik's subchart (source). Disabling the subchart makes the linter pass without warnings or errors:

$ helm lint helm/reana -f helm/configurations/values-dev.yaml --set traefik.enabled=false
==> Linting helm/reana

1 chart(s) linted, 0 chart(s) failed