s3:winbind: Fix idmap_ad creating an invalid local krb5.conf

s3:winbind: Fix idmap_ad creating an invalid local krb5.conf

In case of a trusted domain, we are providing the realm of the primary
trust but specify the KDC IP of the trusted domain. This leads to
Kerberos ticket requests to the trusted domain KDC which doesn't know
about the machine account. However we need a ticket from our primary
trust KDC.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15653

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(backported from commit 8989aa47b7493e6b7978c2efc4a40c781e9a2aee)