For Kerberos testing, disable reverse DNS lookup

For Kerberos testing, disable reverse DNS lookup

In our Kerberos test suite, there isn't much need to worry about the
normal canonicalization that Kerberos provides by looking up the reverse
DNS for the IP address connected to, and in some cases it can actively
cause problems (eg: a captive portal wifi where the normally not
resolvable localhost address used ends up being resolved anyway, and
not to the domain we are using for testing, causing the entire
regression test to fail with errors about not being able to get a TGT
for the remote realm for cross-realm trust).

Therefore, disable it by adding rdns = false into the krb5.conf that's
generated for the test.

Reviewed-By: Heikki Linnakangas
Discussion: https://postgr.es/m/Y/QD2zDkDYQA1GQt@tamriel.snowman.net