Allow forcing plugins with VAGRANT_FORCE_PLUGINS

lib/vagrant/pre-rubygems.rb

@@ -5,11 +5,18 @@
 if defined?(Bundler)
   require "bundler/shared_helpers"
   if Bundler::SharedHelpers.in_bundle?
+    if ENV["VAGRANT_FORCE_PLUGINS"]
+      puts "Vagrant appears to be running in a Bundler environment. Normally,"
+      puts "plugins would not be loaded, but VAGRANT_FORCE_PLUGINS is enabled,"
+      puts "so they will be."
+      puts
+    else
       puts "Vagrant appears to be running in a Bundler environment. Plugins"
       puts "will not be loaded and plugin commands are disabled."
       puts
       ENV["VAGRANT_NO_PLUGINS"] = "1"
     end
+  end
 end
 
 require_relative "bundler"