stack/erp5: modernize haproxy config

* expect a minimum connection_count of 4, using 1 (or 2) seems to cause
balancing issues (test_balancer had several errors).

* set a timeout server, a bit more than the timeout client, to prevent
this warning:

    [WARNING]  (26864) : config : missing timeouts for proxy 'family_default'.
       | While not properly invalid, you will certainly encounter various problems
       | with such a configuration. To fix this, please ensure that all following
       | timeouts are set to a non-zero value: 'client', 'connect', 'server'.

* update health check directive to what the examples from
https://www.haproxy.com/blog/how-to-enable-health-checks-in-haproxy/