helm-chart: add traefik.installTraefik config to opt-out when traefik is installed separately

[gardleopard]

This makes traefik installation optional.

Installation of CRDs happens if traefik is set to not be installed as well. Helm has built in functionality for not installing CRDs with --skip-crds, however for this project that is suboptimal, because it also blocks the install of daskclusters.gateway.dask.org CRD.

Relates to #855

[consideRatio]

I figure the way to use dask-gateway with a standalone traefik install, one would install the dask-gateway specific CRDs manually, and then do helm install .... --skip-crd.

# when installing dask-gateway without traefik
kubectl apply --server-side --force-conflicts -f https://raw.githubusercontent.com/dask/dask-gateway/<VERSION HERE>/resources/helm/dask-gateway/crds/daskclusters.yaml
helm install .... --skip-crd

# when at a later time upgrading without dask-gateway's traefik
kubectl apply --server-side --force-conflicts -f https://raw.githubusercontent.com/dask/dask-gateway/<VERSION HERE>/resources/helm/dask-gateway/crds/daskclusters.yaml
helm upgrade ....

The change in this PR seems fine and could be merged without docs as well, but could you skim through docs to see if there is a place where it's relevant to mention installing or not installing traefik @gardleopard?

[consideRatio]

This is often named enabled in Helm charts where a sub-resource is optional, could we rename it to that? What do you think?

[gardleopard]

I am not opposed to doing it that way. The reason I chose to be more specific was because we use ingressroute and middleware in the gateway templates. So it seems a bit wierd to me that we will use and depend on traefik if traefik is set to enabled: false. What do you think?

[consideRatio]

Ah great point, lets keep using install here!

Do you see a value of naming it as it currently is with traefik.installTraefik over traefik.install? Lets go with what you decide upon among these!

[consideRatio]

Lets go for a merge, this is a detail of little importance - feel free to open a PR if you think it should be changed, I'll go for a merge!

[gardleopard]

How the docs render

[consideRatio]

Thank you @gardleopard!!

[jabbera]

For anyone who stumbles across this the appropriate settings for a default installation of traefik in a namespace called traefik is:

DASK_GATEWAY__ADDRESS: http://proxy-public/services/dask-gateway
DASK_GATEWAY__PROXY_ADDRESS: gateway://traefik.traefik.svc.cluster.local:80

jhub helm chart:

hub:
  consecutiveFailureLimit: 0 # Disable 5 consecutive failurs diables hub
  services:
    dask-gateway:
      url: http://traefik.traefik.svc.cluster.local
      apiToken: <snip>