GCP: Implement configurable snapshot location

[ctrox]

This adds a new config parameter to the VolumeSnapshotLocation to set the location where GCP persistent volume snapshots are stored. I had to update the vendored google API as the field was only recently added. Also the field does not take multiple values as thought before. If you specify more than one the API responds with the following message:

googleapi: Error 413: Value for field 'snapshot.storageLocations' is too large: maximum size 1 element(s); actual size 2., fieldSizeTooLarge

The field just takes any location documented here.

Fixes #1361

[skriss]

Thanks @ctrox, this looks good on first glance! Were you able to successfully test both a backup and a restore?

[ctrox]

Thanks @ctrox, this looks good on first glance! Were you able to successfully test both a backup and a restore?

Yes, I have tested several scenarios. For example a backup of a disk in europe-west6 to us-central and restore that again to a new disk in europe-west6-c works fine.

Just noticed that I missed adding the storageLocation for regional disks, that is now also done.

[skriss]

LGTM. Could you squash this into two commits - one for the dep updates, and then one with the actual velero code changes? thanks!

@nrb @carlisia pls review/merge when ready.