Feature/google create edge cases

fixes #516 

- [x] persist providerid to result immediately to ensure its there for any delete request
- [x] attempt to use existing disks of identical name/specs (primarily for retry use-case)
- [x] protect against bad values of data_disk_size_gb
I know that this isn't part of your changes, but it showed up in the diff, so....

Either `fields['google_data_disk_size_gb'].to_i` before passing it, so we're sure that create_disk gets an integer, or convert to integer within create_disk.