kube-apiserver: use SO_REUSEPORT when creating listener on Unix systems

So multiple instances of kube-apiserver can bind on the same address and
port, to provide seamless upgrades.

Signed-off-by: Mateusz Gozdek <mateusz@kinvolk.io>

test/integration/etcd/server.go

@@ -65,7 +65,7 @@ func StartRealMasterOrDie(t *testing.T, configFuncs ...func(*options.ServerRunOp
 		t.Fatal(err)
 	}
 
-	listener, _, err := genericapiserveroptions.CreateListener("tcp", "127.0.0.1:0")
+	listener, _, err := genericapiserveroptions.CreateListener("tcp", "127.0.0.1:0", net.ListenConfig{})
 	if err != nil {
 		t.Fatal(err)
 	}