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

core/dnsserver/register_test.go

@@ -0,0 +1,31 @@
+package dnsserver
+
+import (
+	"testing"
+)
+
+func TestHandler(t *testing.T) {
+	tp := testPlugin{}
+	c := testConfig("dns", tp)
+	if _, err := NewServer("127.0.0.1:53", []*Config{c}); err != nil {
+		t.Errorf("Expected no error for NewServer, got %s", err)
+	}
+	if h := c.Handler("testplugin"); h != tp {
+		t.Errorf("Expected testPlugin from Handler, got %T", h)
+	}
+	if h := c.Handler("nothing"); h != nil {
+		t.Errorf("Expected nil from Handler, got %T", h)
+	}
+}
+
+func TestHandlers(t *testing.T) {
+	tp := testPlugin{}
+	c := testConfig("dns", tp)
+	if _, err := NewServer("127.0.0.1:53", []*Config{c}); err != nil {
+		t.Errorf("Expected no error for NewServer, got %s", err)
+	}
+	hs := c.Handlers()
+	if len(hs) != 1 || hs[0] != tp {
+		t.Errorf("Expected [testPlugin] from Handlers, got %v", hs)
+	}
+}