[google|compute] disks.all shouldn't return nil

* disk.all should return empty Fog::Compute::Google::Disks
[google|compute] fix typo on disk example
[google|compute] fix typo on disk example
[google|compute] zone in disks.get is now optional
Merge commit 'd26b14fb7be776bde393af59d59b86e60cc95d52' into foreman