IPv6: add scenarios verifying that DAD is done and honored with ipv6.may-fail no

One scenario handles address given by DHCPv6, other by SLAAC.

A convenience step is added as well for loading nftables configuration at once (this could be made nicer if it is possible to use pyroute2.nftables and pyroute2.nfs at the same time somehow) and printing of nftables ruleset in each network NS in dump_status() - solves part of #49 (closed).