Add uploaded keypair support

If the [user has already uploaded a keypair to cloud servers](http://docs.rackspace.com/servers/api/v2/cs-devguide/content/ServersKeyPairs-d1e2545.html), the `key_name` option can be passed as part of the server create blob to Rackspace to use that uploaded public key.  This is useful since long keys may not be able to be uploaded via the personality option (since the max length for the base64 encoded personality file is 256).

[Fog does support this feature](https://github.com/fog/fog/blob/master/lib/fog/rackspace/requests/compute_v2/create_server.rb#L62), although the parameter gets passed to it as a `:keypair`, as opposed to `key_name` or `key_pair`.