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