providers/google: update schema to use a DefaultFunc

This 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.
Refactoring the resourceInstanceTags func just a little

My eye caught this somewhat un-logic mixed use of ‘vs’ and
‘v.(*schema.Set)’, so thought to make it a little cleaner…
Merge pull request #11 from svanharmelen/f-update-google-provider

Update google provider