Use dhcpcd to activate the interfaces.

    Args:
      interfaces: list of string, the output device names to enable.
      logger: logger object, used to write to SysLog and serial port.

def _Dhcpcd(self, interfaces, logger):
    """Use dhcpcd to activate the interfaces.

    Args:
      interfaces: list of string, the output device names to enable.
      logger: logger object, used to write to SysLog and serial port.
    """
    for interface in interfaces:
      dhcpcd = ['/sbin/dhcpcd']
      try:
        subprocess.check_call(dhcpcd + ['-x', interface])
      except subprocess.CalledProcessError:
        # Dhcpcd not yet running for this device.
        logger.info('Dhcpcd not yet running for interface %s.', interface)
      try:
        subprocess.check_call(dhcpcd + [interface])
      except subprocess.CalledProcessError:
        # The interface is already active.
        logger.warning('Could not activate interface %s.', interface)