Let the fields of the CSV dictate which metrics are returned by the plugin.
Also, made the plugin aggregate the server statuses for active/backup up/down/maint/drain/other in their respective BACKEND line.
This will basically make the root directory a place for storing the
major telegraf interfaces, which will make telegraf's godoc looks quite
a bit nicer. And make it easier for contributors to lookup the few data
types that they actually care about.
closes#564
