bump haproxy_exporter from 0.7.1 to v0.8.0

[Kevin Zhao]

Align with parameters pattern with master branch:

The haproxy_exporter has release container image verison v0.8.0:
https://hub.docker.com/r/prom/haproxy-exporter/tags/

From the version v0.8.0 the parameters will have prefix "--" instead of "-".
For align with upstream, we can bump to this version.
/ # haproxy_exporter --help
usage: haproxy_exporter [<flags>]

Flags:
  -h, --help Show context-sensitive help (also try --help-long and --help-man).
      --web.listen-address=":9101"
                             Address to listen on for web interface and telemetry.
      --web.telemetry-path="/metrics"
                             Path under which to expose metrics.

The old version:
"prom/haproxy-exporter:v0.7.1"
[haproxy_exporter, "-haproxy.scrape-uri=unix:/haproxy/haproxy.sock", "-web.listen-address=:{{ $localVals.prometheusPort}}"]

[Kevin Zhao (kevin-zhao)]

Patch to fix this issue:
https://review.openstack.org/#/c/525473