HAProxy: version misaligned between rhel-based and debian-based OSes

[ralgozino]

The version of haproxy installed by the HAProxy role is different in Debian-based OSes than on RHEL-based OSes.

For Debian-based the role is using repos that provide 2.6 LTS version of HAproxy, but for RHEL-based OSes we use the default repository that provides 1.8 versions. v2 versions of HAproxy are different feature wise than v1, for example the prometheus exporter service is not available in 1.8.

Versions must ideally be the same in both OSes.

Reported by @stefanoghinelli