Add local option stub to allow Vagrantfile loading

plugins/commands/plugin/command/list.rb

@@ -9,6 +9,9 @@ module VagrantPlugins
         def execute
           opts = OptionParser.new do |o|
             o.banner = "Usage: vagrant plugin list [-h]"
+
+            # Stub option to allow Vagrantfile loading
+            o.on("--local", "Include local plugins"){|_|}
           end
 
           # Parse the options