Removing usage of the gRPC beta subpackage.

[dhermes]

Fixes#2130.

Most noteworthy is that the "stable" gRPC doesn't have __exit__ and __enter__ and that the stubs are actual classes (instead of some instance of a private and esoteric class).

When scanning for the case-insenstive string grpc I also found some things to change in other parts of the codebase I came across (e.g. a virtualenv root path falling back to protoc)

---

@nathanielmanistaatgoogle PTAL

In particular we went from the public exception type grpc.framework.interfaces.face.face.AbortionError to the private grpc._channel._Rendezvous.

How are we or users supposed to build in error handling? Should we catch grpc.Error (which _Rendezvous inherits from)?

BIG WORRY: Every system tests finishes with

E0819 12:40:27.346506119 22315 network_status_tracker.c:48] Memory leaked as all network endpoints were not shut down

[dhermes]

We'll also now need to update https://github.com/GoogleCloudPlatform/google-cloud-python-happybase

[dhermes]

@nathanielmanistaatgoogle It's also worth noting that GAX is still using the beta API /cc @bjwatson

I realized that and had to modify google.gax.grpc.exc_to_code to allow for _Rendezvous exceptions

[bjwatson]

I created googleapis/gax-python#126 to track this.

[tseaver]

Did we ditch the timeouts because the GA API no longer supports them? I like the simplicity on our side, but worry that users might still need them.

[tseaver]

LGTM