tests: Work auth_log CLIENT_IP out from config instead of env var

tests: Work auth_log CLIENT_IP out from config instead of env var

Instead of passing the CLIENT_IP to the auth_log tests, we can just
work out the source-IP that the client will use from its smb.conf file.

This only works for auth_log_pass_change, but not auth_log.py - the
latter still needs to be run on the :local testenv for other reasons, so
it doesn't use the client.conf. However, we can still update the base
code to use the client.conf IP, as auth_log.py overrides
self.remoteAddress anyway.

The main advantage of this change is it avoids having hardcoded IP
addresses in the selftest framework.

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