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-kafka.go

@@ -20,6 +20,7 @@ import (
 	"errors"
 	"fmt"
 	"io/ioutil"
+	"net"
 
 	"github.com/Sirupsen/logrus"
 
@@ -47,6 +48,12 @@ func (k *kafkaNotify) Validate() error {
 	if len(k.Brokers) == 0 {
 		return errors.New("No broker specified")
 	}
+	// Validate all specified brokers.
+	for _, brokerAddr := range k.Brokers {
+		if _, _, err := net.SplitHostPort(brokerAddr); err != nil {
+			return err
+		}
+	}
 	return nil
 }