organise metrics by plugin instance

[mfournier]

This basically adds an additional level of "hierarchy" to the haproxy metrics in graphite, to separate them based on the "server" settings defined in haproxy.cfg. ie:

• before: collectd.<hostname>.haproxy.gauge-main_website_session_current
• after: collectd.<hostname>.haproxy-main_website.gauge-session_current

...for an haproxy config such as:

backend website
  balance roundrobin
  server  main     1.2.3.4:80 check inter 5000 downinter 500
  server  failover  5.6.7.8:80 check inter 5000 backup