Make experimental a runtime flag

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

client/ping.go

@@ -0,0 +1,19 @@
+package client
+
+import "golang.org/x/net/context"
+
+// Ping pings the server and return the value of the "Docker-Experimental" header
+func (cli *Client) Ping(ctx context.Context) (bool, error) {
+	serverResp, err := cli.get(ctx, "/_ping", nil, nil)
+	if err != nil {
+		return false, err
+	}
+	defer ensureReaderClosed(serverResp)
+
+	exp := serverResp.header.Get("Docker-Experimental")
+	if exp != "true" {
+		return false, nil
+	}
+
+	return true, nil
+}