selftest: Try to tie dns_hub IP mapping to Samba.pm better

selftest: Try to tie dns_hub IP mapping to Samba.pm better

dns_hub.py maps the testenv realm to an IP and Samba.pm maps the testenv
NetBIOS name to an IP. We need to keep the two places consistent, as we
add or remove testenvs.

This patch changes dns_hub.py so that it uses a similar hashmap to
Samba.pm. We now have a hashmap with the same name in 2 different
places, so hopefully that's easier to tie them together and keep them in
sync.

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