REGTESTS: Fix prometheus script to perform HTTP health-checks

REGTESTS: Fix prometheus script to perform HTTP health-checks

TCP Health-checks are enabled on server "s2". However it expects to receive
an HTTP requests. So HAProxy configuration must be changed to perform HTTP
health-checks instead. Otherwise, depending on the timing, an error can be
triggered if a check is performed before the end of the script.

This scripts never failed because TCP_QUICKACK was disabled, adding some
latency on health-checks. But since the last fix, it is an issue.

This patch should be backported as far as 2.4.