Allow kubernetes discoverer to find gRPC destinations

[AndrooHan]

Summary
I updated the kubnetes discoverer to look for veneur global containers with a grpc port name. This will allow us to specify that we are using gRPC.
Additionally, I removed the http:// prefix in the saved podIp because it hits a "too many colons in address" error with gRPC. This will still work with forwarding with http because in proxy.go, the doPost method will check and append http:// if its missing.

Motivation
In Kubernetes we want to proxy metrics to veneur global with gRPC. This edit in our fork fixed the issues we were hitting.

Test plan
Ran this in our veneur-proxy pods that utilize this Kubernetes discoverer code with gRPC to verify everything works.

Rollout/monitoring/revert plan
This change should be backwards compatible as the doPost function for http communication prepends the necessary prefix. This only affects gRPC destinations used by proxysrv which shouldn't have been available with kubernetes.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}