selftest: Map realm to IP address (instead of iface)

selftest: Map realm to IP address (instead of iface)

The code is more readable if the hashmap translates between realm and
DC-name, rather than realm-to-iface. We already have a function to map
between DC-name and iface (and since we're doing this, we might as well
map straight to IP address).

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