stack/erp5: serve balancer requests when client certificate is not verified

We configure haproxy with "verify optional", which makes haproxy request
a client certificate, but accept the case where client does not present
a certificate, but as described in [1], if client present a certificate
and this certificate can not be verified, handshake is aborted. This is
not what we want, we want to treat the case of a non verified
certificate same as the case of the absence of certificate.

This configures haproxy accordingly, using "crt-ignore-err all" to allow
handshake anyway.

Once this was fixed, there was a remaining problem with
client_cert_verified acl, haproxy acl are OR, but this rule was supposed
to be a AND (client present a certificate AND it is verified), this was
rewritten to use inline condition which are AND.

[1]: https://cbonte.github.io/haproxy-dconv/1.8/configuration.html#5.1-verify

Also adjust test_x_forwarded_for_stripped_when_no_certificate to assert
that there is no X-Forwarded-For header at all when no client
certificate.