install: do not assume /etc/krb5.conf.d exists

Add `includedir /etc/krb5.conf.d` to /etc/krb5.conf only if
/etc/krb5.conf.d exists.

Do not rely on /etc/krb5.conf.d to enable the certauth plugin.

This fixes install on platforms which do not have /etc/krb5.conf.d.

https://pagure.io/freeipa/issue/6589

Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>

.gitignore

@@ -77,7 +77,6 @@ freeipa2-dev-doc
 /daemons/dnssec/ipa-ods-exporter.socket
 /daemons/ipa-kdb/ipa_kdb_tests
 /daemons/ipa-kdb/tests/.dirstamp
-/daemons/ipa-kdb/ipa-certauth
 /daemons/ipa-otpd/ipa-otpd
 /daemons/ipa-otpd/ipa-otpd.socket
 /daemons/ipa-otpd/ipa-otpd@.service