Query for type "compute" instead of name "nova"

The former doesn't work in my private OpenStack cloud (from Metacloud), because the service is not named "nova"; it's named "Compute Service".

    {
        Name:      "Compute Service",
        Type:      "compute",
        ...
    }

As a result of this, I was getting this error from gophercloud while trying to run the Packer OpenStack builder:

```
Missing endpoint, or insufficient privileges to access endpoint
```

I think type "compute" is probably a more portable way to do it. That was the impression I got from looking at
http://docs.rackspace.com/auth/api/v2.0/auth-client-devguide/content/Service_Types-d1e265.html

Cc: @sam-falvo, @jrperritt, @mitchellh