s3-libnet_join: allow fallback to NTLMSSP auth in libnet_join

s3-libnet_join: allow fallback to NTLMSSP auth in libnet_join

When a non-DNS and non-default admin domain is provided during the join
sometimes we might not be able to kinit with 'user@SHORTDOMAINNAME'
(e.g. when the winbind krb5 locator is not installed). In that case lets
fallback to NTLMSSP, like we do in winbind.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13861

Guenther

Signed-off-by: Guenther Deschner <gd@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Wed Apr  3 18:57:31 UTC 2019 on sn-devel-144