Fix disk cleanup corner-case

[Temikus]

If fog file specifies just a test namespace, disk cannot be properly cleaned up:

test:
  google_project: my-proj
  google_json_key_location: /my-project-key.json

Since disk is not actually instantiated, it doesn't have a name property:

==> default:  -- Additional Disks:[]
...
/Users/temikus/.vagrant.d/gems/2.4.4/gems/vagrant-google-2.4.0/lib/vagrant-google/action/run_instance.rb:269:in `block in call': undefined method `name' for #<Hash:0x0000000100c489b0> (NoMethodError)