Instead of hardcoding plugin lists in autopath/health, use interfaces. (#1306)

Switched health and autopath plugin to allow any plugins to be used instead
of a hardcoded list. I did not switch federation over since it wasn't
obvious that anything other than kubernetes could be used with it.

Fixes #1291

plugin/health/healther.go

@@ -36,9 +36,3 @@ func (h *health) poll() {
 	}
 	h.SetOk(true)
 }
-
-// Plugins that implements the Healther interface.
-var healthers = map[string]bool{
-	"erratic":    true,
-	"kubernetes": true,
-}