Add `--format` flag for `docker plugin ls`

This fix tries to address the enhancement discussed in 28735 to add
`--format` for the output of `docker plugin ls`.

This fix
1. Add `--format` and `--quiet` flags to `docker plugin ls`
2. Convert the current implementation to use `formatter`, consistent with
   other docker list commands.
3. Add `pluginsFormat` for config.json.

Related docs has been updated.

Several unit tests have been added to cover the changes.

This fix is related to 28708 and 28735.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docs/reference/commandline/cli.md

@@ -131,6 +131,12 @@ Docker's client uses this property. If this property is not set, the client
 falls back to the default table format. For a list of supported formatting
 directives, see the [**Formatting** section in the `docker images` documentation](images.md)
 
+The property `pluginsFormat` specifies the default format for `docker plugin ls` output.
+When the `--format` flag is not provided with the `docker plugin ls` command,
+Docker's client uses this property. If this property is not set, the client
+falls back to the default table format. For a list of supported formatting
+directives, see the [**Formatting** section in the `docker plugin ls` documentation](plugin_ls.md)
+
 The property `serviceInspectFormat` specifies the default format for `docker
 service inspect` output. When the `--format` flag is not provided with the
 `docker service inspect` command, Docker's client uses this property. If this
@@ -186,6 +192,7 @@ Following is a sample `config.json` file:
       },
       "psFormat": "table {{.ID}}\\t{{.Image}}\\t{{.Command}}\\t{{.Labels}}",
       "imagesFormat": "table {{.ID}}\\t{{.Repository}}\\t{{.Tag}}\\t{{.CreatedAt}}",
+      "pluginsFormat": "table {{.ID}}\t{{.Name}}\t{{.Enabled}}",
       "statsFormat": "table {{.Container}}\t{{.CPUPerc}}\t{{.MemUsage}}",
       "serviceInspectFormat": "pretty",
       "detachKeys": "ctrl-e,e",

docs/reference/commandline/plugin_ls.md

@@ -24,8 +24,10 @@ Aliases:
   ls, list
 
 Options:
+      --format string   Pretty-print plugins using a Go template
       --help            Print usage
       --no-trunc        Don't truncate output
+  -q, --quiet           Only display plugin IDs
 ```
 
 Lists all the plugins that are currently installed. You can install plugins
@@ -40,6 +42,32 @@ ID                  NAME                             TAG                 DESCRIP
 69553ca1d123        tiborvass/sample-volume-plugin   latest              A test plugin for Docker   true
 ```
 
+## Formatting
+
+The formatting options (`--format`) pretty-prints plugins output
+using a Go template.
+
+Valid placeholders for the Go template are listed below:
+
+Placeholder    | Description
+---------------|------------------------------------------------------------------------------------------
+`.ID`          | Plugin ID
+`.Name`        | Plugin name
+`.Description` | Plugin description
+`.Enabled`     | Whether plugin is enabled or not
+
+When using the `--format` option, the `plugin ls` command will either
+output the data exactly as the template declares or, when using the
+`table` directive, includes column headers as well.
+
+The following example uses a template without headers and outputs the
+`ID` and `Name` entries separated by a colon for all plugins:
+
+```bash
+$ docker plugin ls --format "{{.ID}}: {{.Name}}"
+4be01827a72e: tiborvass/no-remove
+```
+
 ## Related information
 
 * [plugin create](plugin_create.md)