organise metrics by plugin instance

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
```