provider/google: fix acc tests and updated the provider schema to use a DefaultFunc

Fixed the acc tests and made them executable without having to first manually specify a disk in GCE. Wondering a little when these tests where last run successfully… Must be some time ago considering what I needed to fix here :wink:

Updating the schema makes testing easier and gives you a way to configure the provider using env variables. It also makes the provider more inline following the TF 0.2 approach.