Revisit gRPC's required usage of plaintext

[ljnelson]

In setting up the gRPC communication, I seem to recall there is a method call that sets up plaintext communication. Obviously this needs to be locked down; my recollection is that merely removing this method doesn't do the trick. Needs more research, particularly given that this project doesn't "own" the server (Tiller).

[ljnelson]

Technically we're OK because you can always pass in your own ManagedChannel built however you like, but this would be a pretty common subclass operation I'd think.