Align the disk interface more closely with the API.

The caller need only specify either image_name OR snapshot + size
when creating a disk in a given zone.

This patch also removes the @default_zone variable being referenced
in several places (because it is not actually defined anywhere).
Merge pull request #2064 from maginatics/gce_insert_disk

[google|compute] Accept/propagate the correct params when creating a new disk