dht: IPv6 support for ICE public address

This patch uses the fist IPv6 address given by DHT nodes as public address
used for ICE negotiation.
This address is used in priority, overloading any previous one registered,
in particular over IPv4 one.

Change-Id: Ie3b3d9edcfe61871adcb87b965860530ab4f1d31
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>