notifiers: Stop using url.Parse in validating address format. (#4011)

url.Parse() wrongly parses an address of format "address:port"
which is fixed in go1.8.  This inculcates a breaking change
on our end. We should fix this wrong usage everywhere so that
migrating to go1.8 eventually becomes smoother.

cmd/notify-nats.go

@@ -18,6 +18,7 @@ package cmd
 
 import (
 	"io/ioutil"
+	"net"
 
 	"github.com/Sirupsen/logrus"
 	"github.com/nats-io/go-nats-streaming"
@@ -52,7 +53,7 @@ func (n *natsNotify) Validate() error {
 	if !n.Enable {
 		return nil
 	}
-	if _, err := checkURL(n.Address); err != nil {
+	if _, _, err := net.SplitHostPort(n.Address); err != nil {
 		return err
 	}
 	return nil