openstack builder error out when gets blank keypair

As I mentioned in https://github.com/mitchellh/packer/pull/1405, somehow gophercloud ends up calling the wrong URL for `os-keypairs` -- it calls the Volume API, which returns a 404, which results in packer getting a blank keypair but packer keeps proceeding and then fails at a later place.

It seems like it should fail immediately when it gets the blank keypair.