[rackspace|compute_v2] Fixed issue adding networks to new servers

This pull request is an attempt to address adding networks to servers during there creation.

Both pull request https://github.com/fog/fog/pull/1761 and pull request https://github.com/fog/fog/pull/1578 have attempted to fix this by the adding  a `networks` attribute. @bradgignac has suggested against this approach because the attribute would only be used during the creation process and it would always return `nil` if user would call `server.networks`.

 This pull request thats the approach of accessing the networks using the `attributes` hash.