Use GRPC for management APIs to complement HTTP offering

[robskillington]

Since a lot of our tooling/systems are now exposing their APIs as GRPC instead of HTTP we should align and change the HTTP tooling for health check, resigning, etc to use GRPC.

This gives us type safe clients and access to our APIs across all other platforms.

We should also offer HTTP, but in a way that's secondary and automatically generated. One approach is to extract the reflect based HTTP offering from M3DB and place that on top of the GRPC service:
https://github.com/m3db/m3db/blob/a4b5961506a8dc423853b4809998717995ac3db9/network/server/httpjson/handlers.go

The other is potentially offering a GRPC -> HTTP proxy such as the Extensible Service Proxy written for GCP:
https://github.com/cloudendpoints/esp

The first approach is more "batteries included" and what I think would be the preferred approach at the this current point in time.