[nats helm] allow the nats routes to drop the namespace when useFQDN is false

[NickLarsenNZ]

It would be quite nice to drop the namespace from the short name in the routes.

https://github.com/nats-io/k8s/blob/112d3eb9269e78abd6af91a90ae076f63be5f0e3/helm/charts/nats/templates/_helpers.tpl#L93

I'd like to create certs for the cluster with the short names as it is only used in a single namespace (bar an external name for remote access during a migration).

I'm not sure anyone else would want this, so I haven't jumped in and create a PR yet, and will hard code the namespace into my certificates for now. If there is interest, I'd be happy to raise the PR.

The two options I see:

1. Simply drop the namespace from the short name routes (potentially breaking changes for TLS, or multi-namespace, same-cluster setups).
2. Add an option to toggle the inclusion of the namespace for the short name (defaulting to true to avoid a breaking change).

[caleblloyd]

Is there a "best practice" in k8s for FQDN or no FQDN? I know that using an FQDN with a trailing . can guarantee a single DNS lookup, and no propagation to the external resolver.

[caleblloyd]

Looks like we don't even have the trailing . with useFQDN - we should add that

I would be OK with dropping the namespace from useFQDN: false. It could break users that had useFQDN: false with TLS certificates, although I do not think that would be a very common case. But we would still want to call it out in the release notes