Move gcloud.storage.iterator into gcloud's common package

[jgeewax]

We'll need this for Cloud DNS in the ManagedZones.list method (https://developers.google.com/cloud-dns/api/v1beta1/managedZones/list)

[aliafshar]

Also, any other API listing will require it.

[dhermes]

Closing. We'll address this if / when it comes up, but YAGNI for now.

[dhermes]

Relevant again due to #933.