sip_nat_settings: Update script for latest Linux.

With the latest Linux, 'ifconfig' is not installed on default anymore.
Furthermore, the output of the current net-tools 'ifconfig' changed.
Therefore, parsing failed. This update uses 'ip addr show' instead.
Finally, the service for the external IP changed.

Change-Id: I9b1a7c3f457e3553b50a3e9a55524e40d70245a0