Improved HAProxy collector.

More robust HAProxy collector with simpler config

The HAProxy collector is made more robust in the face of possible
HAProxy output changes by using a proper CSV parsing library, rather
than relying on indices in a list.

It is easier to configure for additional values that one may wish to
report by simply providing the names of the metrics to report as a list,
defined at the top of the file.

Reduced number of default metrics to report

List now matches the previous HAProxy collector's list of metrics that
were reported.

Bug fixes

Improved checking to ensure that we get only the lines we want
from our output, presence of headers is handled better, and much
more.