selftest: Restore IPs 12-16 for selftest client

selftest: Restore IPs 12-16 for selftest client

The assumption that tests only used the .11 IP was wrong. The
winsreplication test tries to use multiple different IPs - CI doesn't
fail when we remove the additional IPs, but it starts to skip test
cases.

+ Update get_interfaces_config() and get_ipv4_addr() so we can add
multiple different IPs for the same host.
+ Update selftest.pl so the client gets 6 IP addresses.
+ Update comments to better reflect this dependency.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>