CVE-2020-25717: s3:auth: don't let create_local_token depend on !winbind_ping()

CVE-2020-25717: s3:auth: don't let create_local_token depend on !winbind_ping()

We always require a running winbindd on a domain member, so
we should better fail a request instead of silently alter
the behaviour, which results in a different unix token, just
because winbindd might be restarted.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14801
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14556

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>