code: func (c *Client) Authenticate(d *openstack.Driver) error {
  	if c.Provider != nil {
  		return nil
  	}
  
  	log.Debug("Authenticating to Rackspace.", map[string]string{
  		"Username": d.Username,
  	})
  
  	apiKey := c.driver.APIKey
  	opts := gophercloud.AuthOptions{
  		Username: d.Username,
  		APIKey:   apiKey,
  	}
  
  	provider, err := rackspace.NewClient(rackspace.RackspaceUSIdentity)
  	if err != nil {
  		return err
  	}
  
  	provider.UserAgent.Prepend(fmt.Sprintf("docker-machine/v%d", version.APIVersion))
  
  	err = rackspace.Authenticate(provider, opts)
  	if err != nil {
  		return err
  	}
  
  	c.Provider = provider
  
  	return nil
  }
docs: // Authenticate creates a Rackspace-specific Gophercloud client.