firewall_rules.py: fix outgoing ipv6 test
firewall_rules.py: fix outgoing ipv6 test

Previously, the test assumed only 1 ipv6 connection for the host.
Now, it gets the ipv6 address assigned to the interface with
$SERVER_IP.

daweiwu/meta-iotqa-1,ostroproject/meta-iotqa,ostroproject/meta-iotqa,ostroproject/meta-iotqa,daweiwu/meta-iotqa-1,daweiwu/meta-iotqa-1,wanghongjuan/meta-iotqa-1,wanghongjuan/meta-iotqa-1,daweiwu/meta-iotqa-1,wanghongjuan/meta-iotqa-1,wanghongjuan/meta-iotqa-1,ostroproject/meta-iotqa,ostroproject/meta-iotqa,daweiwu/meta-iotqa-1,wanghongjuan/meta-iotqa-1