selftest: Pass realm-to-IP mapping to dns_hub as an argument

selftest: Pass realm-to-IP mapping to dns_hub as an argument

Instead of storing hashmaps in 2 different files, we can just convert a
perl hashmap into a string, pass it to dns_hub, and convert it back into
a python dictionary.

The main reason for doing this is the IP-to-testenv mapping now all
lives in a single file (Samba.pm). All this logic is right next to each
other rather than being split across multiple files. Hopefully this will
make it easier to keep it up to date as we add new testenvs.

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