change service port name to support Istio explicit protocol selection
change service port name to support Istio explicit protocol selection

Kubernetes has a native construct for this called `appProtocol`. Kubernetes
expects either a IANA protocol or a custom protocol prefix with a domain.

I tried setting `appProtocol` to `nats.io/client` but that does not effect
in Istio, as istio expects only these protocols:
https://istio.io/latest/docs/ops/configuration/traffic-management/protocol-selection/#explicit-protocol-selection

Setting `appProtocol` to `tcp` probably does work, but tcp is not a valid
iana service name. See:
https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml

Therefor the least intrusive, compatible leaves us with prefixing the service
port name.

Fixes: #88

nats-io/nats-operator,nats-io/nats-operator