rgw: fail s3 POST auth if keystone not configured

rgw: fail s3 POST auth if keystone not configured

Fixes: #10698
This fixes issue introduced in 8b3dfc9472022ea45ad24e02e0aa21dfdad798f8,
where if user does not exist, we try keystone authentication. However,
if keystone is not configured we justt fall through without failing.
This would have failed later on due to bucket permissions, unless bucket
had a public write permissions.

Backports: Firefly
Reported-by: Valery Tschopp <valery.tschopp@switch.ch>
Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Conflicts:
	src/rgw/rgw_rest_s3.cc