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/autopath/autopath.go

@@ -46,6 +46,12 @@ import (
 // If Func returns a nil slice, no autopathing will be done.
 type Func func(request.Request) []string
 
+// AutoPather defines the interface that a plugin should implement in order to be
+// used by AutoPath.
+type AutoPather interface {
+	AutoPath(request.Request) []string
+}
+
 // AutoPath perform autopath: service side search path completion.
 type AutoPath struct {
 	Next  plugin.Handler