[Openstack|Compute|Volume] Ensure #attach succeeds and lists one attachment

Previously, inspecting the test objects would show 2 attachments per
server for every time the #attach was called.
A better fix might be to change this to something like `attachments + [data.body['volumeAttachment']]` instead of using the `<<` which inadvertently changes the mock data. I suspect that otherwise this will fix the mock values, but also break certain real usage (since the volume model would not be updated after the attach as might be expected from before). Does that make sense? Would you be up for testing that change to see if it also removes duplicates as you expect? Thanks!