Re-do #5482 w/ fixes for 'master' and out-of-repo PR builds. (#5485)
Re-do #5482 w/ fixes for 'master' and out-of-repo PR builds. (#5485)


tseaver/google-cloud-python,dhermes/google-cloud-python,googleapis/google-cloud-python,jonparrott/gcloud-python,GoogleCloudPlatform/gcloud-python,tseaver/google-cloud-python,tseaver/gcloud-python,dhermes/google-cloud-python,tseaver/google-cloud-python,jonparrott/gcloud-python,jonparrott/google-cloud-python,tswast/google-cloud-python,dhermes/google-cloud-python,tseaver/gcloud-python,tswast/google-cloud-python,dhermes/gcloud-python,jonparrott/google-cloud-python,tswast/google-cloud-python,googleapis/google-cloud-python,dhermes/gcloud-python,GoogleCloudPlatform/gcloud-python