Rationalize tox vs. gRPC:
Rationalize tox vs. gRPC:

- Install 'grpcio' and related deps by default, and omit adding '_testing'
  to PYTHONPATH.

- For Python 2.6, and lint, RTD, suppress installing gRPC-related deps,
  and add '_testing' to PYTHONPATH.

elibixby/gcloud-python,dhermes/gcloud-python,dhermes/google-cloud-python,jonparrott/gcloud-python,calpeyser/google-cloud-python,dhermes/gcloud-python,tseaver/gcloud-python,calpeyser/google-cloud-python,jonparrott/gcloud-python,googleapis/google-cloud-python,googleapis/google-cloud-python,tartavull/google-cloud-python,Fkawala/gcloud-python,dhermes/google-cloud-python,jonparrott/google-cloud-python,jonparrott/google-cloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,GoogleCloudPlatform/gcloud-python,Fkawala/gcloud-python,daspecster/google-cloud-python,jgeewax/gcloud-python,tseaver/google-cloud-python,dhermes/google-cloud-python,tseaver/google-cloud-python,jgeewax/gcloud-python,tswast/google-cloud-python,daspecster/google-cloud-python,quom/google-cloud-python,tartavull/google-cloud-python,tseaver/google-cloud-python,tseaver/gcloud-python,elibixby/gcloud-python,tswast/google-cloud-python,quom/google-cloud-python