plugins: experimental support for new plugin management

This patch introduces a new experimental engine-level plugin management
with a new API and command line. Plugins can be distributed via a Docker
registry, and their lifecycle is managed by the engine.
This makes plugins a first-class construct.

For more background, have a look at issue #20363.

Documentation is in a separate commit. If you want to understand how the
new plugin system works, you can start by reading the documentation.

Note: backwards compatibility with existing plugins is maintained,
albeit they won't benefit from the advantages of the new system.

Signed-off-by: Tibor Vass <tibor@docker.com>
Signed-off-by: Anusha Ragunathan <anusha@docker.com>

cobraadaptor/adaptor.go

@@ -6,6 +6,7 @@ import (
 	"github.com/docker/docker/api/client/image"
 	"github.com/docker/docker/api/client/network"
 	"github.com/docker/docker/api/client/node"
+	"github.com/docker/docker/api/client/plugin"
 	"github.com/docker/docker/api/client/registry"
 	"github.com/docker/docker/api/client/service"
 	"github.com/docker/docker/api/client/swarm"
@@ -81,6 +82,7 @@ func NewCobraAdaptor(clientFlags *cliflags.ClientFlags) CobraAdaptor {
 		system.NewVersionCommand(dockerCli),
 		volume.NewVolumeCommand(dockerCli),
 	)
+	plugin.NewPluginCommand(rootCmd, dockerCli)
 
 	rootCmd.PersistentFlags().BoolP("help", "h", false, "Print usage")
 	rootCmd.PersistentFlags().MarkShorthandDeprecated("help", "please use --help")

error.go

@@ -0,0 +1,21 @@
+package cli
+
+import "bytes"
+
+// Errors is a list of errors.
+// Useful in a loop if you don't want to return the error right away and you want to display after the loop,
+// all the errors that happened during the loop.
+type Errors []error
+
+func (errs Errors) Error() string {
+	if len(errs) < 1 {
+		return ""
+	}
+	var buf bytes.Buffer
+	buf.WriteString(errs[0].Error())
+	for _, err := range errs[1:] {
+		buf.WriteString(", ")
+		buf.WriteString(err.Error())
+	}
+	return buf.String()
+}