selftest: Add helper function to get interfaces config

selftest: Add helper function to get interfaces config

Add a helper function to return the IPv4/IPv6 addresses for the
smb.conf. This keeps the netmask assumptions in the same places as
the IP subnet assumptions.

This refactor means we no longer need to store $ctx->{interfaces}, as it
was only used in one place.

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