modify krb5.conf to avoid generic failure: GSSAPI Error

taken from https://stackoverflow.com/questions/50250856/how-to-fix-the-active-directory-authentication-issue-with-ubuntu-16-04-in-aws

ad-auth/templates/krb5.conf.j2

@@ -3,6 +3,7 @@ default_realm = {{ domain.upper() }}
 dns_lookup_realm = false
 dns_lookup_kdc = true
 forwardable = true
+rdns = false
 
 [domain_realm]
 .{{ domain }} = {{ domain.upper() }}