notifications: fix expvar for Go 1.7

Remove EndpointConfig.Transport from the return value of the
registry.notifications.endpoints expvar.Func.  It results in an empty
value for that expvar variable under Go 1.7 because it is a non-nil
*http.Transport, which Go 1.7 can no longer encode as JSON.

Signed-off-by: Noah Treuhaft <noah.treuhaft@docker.com>
(cherry picked from commit 9a58c91051)

notifications/endpoint.go

@@ -13,7 +13,7 @@ type EndpointConfig struct {
 	Threshold         int
 	Backoff           time.Duration
 	IgnoredMediaTypes []string
-	Transport         *http.Transport
+	Transport         *http.Transport `json:"-"`
 }
 
 // defaults set any zero-valued fields to a reasonable default.