Expose quasi-enum types for string values w/ choices.
Expose quasi-enum types for string values w/ choices.

Users who want to avoid tripping over string literal mismatches can use
class constants instead.

Addresses:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/1056/files#r37029533

jgeewax/gcloud-python,tseaver/google-cloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,elibixby/gcloud-python,tseaver/google-cloud-python,waprin/gcloud-python,CyrusBiotechnology/gcloud-python,googleapis/google-cloud-python,jgeewax/gcloud-python,thesandlord/gcloud-python,quom/google-cloud-python,dhermes/google-cloud-python,tswast/google-cloud-python,calpeyser/google-cloud-python,VitalLabs/gcloud-python,daspecster/google-cloud-python,EugenePig/gcloud-python,thesandlord/gcloud-python,vj-ug/gcloud-python,jonparrott/google-cloud-python,jonparrott/gcloud-python,daspecster/google-cloud-python,dhermes/gcloud-python,tartavull/google-cloud-python,EugenePig/gcloud-python,jonparrott/gcloud-python,tseaver/gcloud-python,tartavull/google-cloud-python,waprin/google-cloud-python,Fkawala/gcloud-python,waprin/gcloud-python,waprin/google-cloud-python,googleapis/google-cloud-python,calpeyser/google-cloud-python,dhermes/google-cloud-python,dhermes/gcloud-python,CyrusBiotechnology/gcloud-python,tseaver/gcloud-python,quom/google-cloud-python,tseaver/google-cloud-python,VitalLabs/gcloud-python,GoogleCloudPlatform/gcloud-python,vj-ug/gcloud-python,tswast/google-cloud-python,elibixby/gcloud-python,GoogleCloudPlatform/gcloud-python,tswast/google-cloud-python,Fkawala/gcloud-python