software/erp5: include timestamp in haproxy error log

Haproxy log errors for events like when all backends from a family fail the
health check, it is helpful for investigations to have a timestamp in this log.

We were using ActionFileDefaultTemplate so that access logs use standard
"combined"-like format, we change so that this apply only to the access log,
so that error log have the default "timestamp hostname process[pid] message"
format.