Reports also from BACKEND and FRONTEND

Due to how OpenTSDB summarizes values from different backend servers
in a non-deterministic manner (if one looks at the very freshest
values coming in, sometimes they do not include values from all
servers at first, but does after a while), it makes good sense to
also get HAProxy's summarized view instead. HAProxy's summary is
always correct. So, we now support getting metrics even from
the FRONTEND and BACKEND sections of the HAProxy stats report.