commands/box: list has machine-readable output

plugins/commands/box/command/list.rb

@@ -30,6 +30,9 @@ module VagrantPlugins
           # and which don't, since we plan on doing that transparently.
           boxes.each do |name, provider, _v1|
             @env.ui.info("#{name.ljust(longest_box_length)} (#{provider})", :prefix => false)
+
+            @env.ui.machine("box-name", name)
+            @env.ui.machine("box-provider", provider)
           end
 
           # Success, exit status 0

website/docs/source/v2/cli/machine-readable.html.md

@@ -73,6 +73,20 @@ with the machine-readable output.
 </tr>
 </thead>
 
+<tr>
+<td>box-name</td>
+<td>
+	Name of a box installed into Vagrant.
+</td>
+</tr>
+
+<tr>
+<td>box-provider</td>
+<td>
+	Provider for an installed box.
+</td>
+</tr>
+
 <tr>
 <td>provider-name</td>
 <td>