core: output available commands in machine-readable

lib/vagrant/cli.rb

@@ -65,6 +65,7 @@ module Vagrant
 
         commands.keys.sort.each do |key|
           o.separator "     #{key.ljust(longest+2)} #{commands[key]}"
+          @env.ui.machine("cli-command", key.dup)
         end
 
         o.separator ""

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

@@ -99,6 +99,13 @@ with the machine-readable output.
 </td>
 </tr>
 
+<tr>
+<td>cli-command</td>
+<td>
+	A subcommand of <code>vagrant</code> that is available.
+</td>
+</tr>
+
 <tr>
 <td>provider-name</td>
 <td>