gcp: add support for regional storage types

[wwitzel3]

The version of the Google API library we have vendored does not have the RegionDisks API.

I've attempted to use 15aea8711db98e640b277da9771a9d74270c2edc of google.golang.org/api but that results in some other compilation issues that I am unsure of how to fix currently.

vendor/google.golang.org/api/internal/creds.go:36:10: undefined: google.CredentialsFromJSON
Makefile:113: recipe for target 'shell' failed
make[2]: *** [shell] Error 2
make[2]: Leaving directory '/home/wwitzel3/go/src/github.com/heptio/ark'
Makefile:97: recipe for target '_output/bin/linux/amd64/ark' failed
make[1]: *** [_output/bin/linux/amd64/ark] Error 2
make[1]: Leaving directory '/home/wwitzel3/go/src/github.com/heptio/ark'
Makefile:67: recipe for target 'all' failed
make: *** [all] Error 2

Once I get the vendoring figured out, I'll add tests, and take this out of work in progress.

Fixes #765

[skriss]

@wwitzel3 guessing you'll also need to bump the rev of golang.org/x/oauth2 that we're using?

[wwitzel3]

I'll give that a shot, thanks.