cli-plugins: disable use of dial-stdio

Since #1654 so far we've had problems with it not working on Windows (npipe
lacked the `CloseRead` method) and problems with using tcp with tls (the tls
connection also lacks `CloseRead`). Both of these were workedaround in #1718
which added a nop `CloseRead` method.

However I am now seeing hangs (on Windows) where the `system dial-stdio`
subprocess is not exiting (I'm unsure why so far).

I think the 3rd problem found with this is an indication that `dial-stdio` is
not quite ready for wider use outside of its initial usecase (support for
`ssh://` URLs to connect to remote daemons).

This change simply disables the `dial-stdio` path for all plugins. However
rather than completely reverting 891b3d953e ("cli-plugins: use `docker system
dial-stdio` to call the daemon") I've just disabled the functionality at the
point of use and left in a trap door environment variable so that those who
want to experiment with this mode (and perhaps fully debug it) have an easier
path do doing so.

The e2e test for this case is disabled unless the trap door envvar is set. I
also renamed the test to clarify that it is about cli plugins.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli-plugins/plugin/plugin.go

@@ -31,7 +31,11 @@ func runPlugin(dockerCli *command.DockerCli, plugin *cobra.Command, meta manager
 	PersistentPreRunE = func(_ *cobra.Command, _ []string) error {
 		var err error
 		persistentPreRunOnce.Do(func() {
-			err = tcmd.Initialize(withPluginClientConn(plugin.Name()))
+			var opts []command.InitializeOpt
+			if os.Getenv("DOCKER_CLI_PLUGIN_USE_DIAL_STDIO") != "" {
+				opts = append(opts, withPluginClientConn(plugin.Name()))
+			}
+			err = tcmd.Initialize(opts...)
 		})
 		return err
 	}