[helm] Don't hard-code .cluster.local in the teleport-cluster chart

[programmerq]

What would you like Teleport to do?

Kubernetes allows cluster administrators to choose a value other than cluster.local. This means that our chart's hard-coded use leads to failures on a Kubernetes cluster that doesn't use the default value.

https://github.com/gravitational/teleport/blob/master/examples/chart/teleport-cluster/templates/_helpers.tpl#L90

% grep -Ri 'serviceFQDN' .
./charts/teleport-operator/templates/_helpers.tpl:{{- $clusterAddr := include "teleport-cluster.auth.serviceFQDN" . -}}
./charts/teleport-operator/templates/_helpers.tpl:{{- define "teleport-cluster.auth.serviceFQDN" -}}{{- end }}
./templates/proxy/_config.common.tpl:  auth_server: "{{ include "teleport-cluster.auth.serviceFQDN" . }}:3025"
./templates/auth/_config.common.tpl:  public_addr: "{{ include "teleport-cluster.auth.serviceFQDN" . }}:3026"
./templates/_helpers.tpl:{{- define "teleport-cluster.auth.serviceFQDN" -}}

A common paradigm in the helm community is to include a clusterDomain value that defaults to cluster.local. An alternate approach would be to avoid using the FQDN and instead rely on the short DNS names (if feasible).

Here's an example in the bitnami etcd chart:

https://github.com/bitnami/charts/blob/main/bitnami/etcd/values.yaml#L49-L51
https://github.com/bitnami/charts/blob/main/bitnami/etcd/templates/statefulset.yaml#L182-L183