IPv6 hostnetwork IPERF3 iperf-server-start not picking the right IPv6 address

[HughNhan]

Currently the ipv6 hostnetwork iperf testcases are not working. The Crucible iperf client is contacting the server using a bogus IPv6 server address. This is is due to a bug in the Crucible iperf-server-start that detemines the server ipv6 address.
https://github.com/perftool-incubator/bench-iperf/blob/main/iperf-server-start#L133 - this code assumes the first entry is the address to use which may not be the case.