Bridge driver to Warn if xt_conntrack not present

Signed-off-by: Alessandro Boch <aboch@docker.com>

libnetwork/drivers/bridge/bridge.go

@@ -133,6 +133,9 @@ func Init(dc driverapi.DriverCallback, config map[string]interface{}) error {
 	if out, err := exec.Command("modprobe", "-va", "nf_nat").CombinedOutput(); err != nil {
 		logrus.Warnf("Running modprobe nf_nat failed with message: `%s`, error: %v", strings.TrimSpace(string(out)), err)
 	}
+	if out, err := exec.Command("modprobe", "-va", "xt_conntrack").CombinedOutput(); err != nil {
+		logrus.Warnf("Running modprobe xt_conntrack failed with message: `%s`, error: %v", strings.TrimSpace(string(out)), err)
+	}
 	if err := iptables.FirewalldInit(); err != nil {
 		logrus.Debugf("Fail to initialize firewalld: %v, using raw iptables instead", err)
 	}