func getPrivateIPInterface() string {
	iface, err := config.GetString("driver:private-ip-interface")
	if err == nil {
		return iface
	}
	return "eth0"
}
getPrivateIPInterface returns the interface name which contains
the private IP address of a machine provisioned with the given
driver. This is a workaround while the Driver interface does not
provide a way to access the instance private IP.
package dm
import (
	"encoding/base64"
	"errors"
	"fmt"
	"io/ioutil"
	"strings"
	"github.com/tsuru/config"
	"github.com/tsuru/tsuru/iaas/dockermachine"
)