core: plugin management requires bundler

With this change, the `Vagrant::plugins_enabled?` is now false when the
embedded Bundler is not available.

Resolve the broken RSpec unit tests after
479323f1e8.

lib/vagrant/shared_helpers.rb

@@ -38,11 +38,11 @@ module Vagrant
     ENV["VAGRANT_INSTALLER_EMBEDDED_DIR"]
   end
 
-  # This returns whether or not 3rd party plugins should be loaded.
+  # This returns whether or not 3rd party plugins should and can be loaded.
   #
   # @return [Boolean]
   def self.plugins_enabled?
-    !ENV["VAGRANT_NO_PLUGINS"]
+    !ENV["VAGRANT_NO_PLUGINS"] && $vagrant_bundler_runtime
   end
 
   # Whether or not super quiet mode is enabled. This is ill-advised.

test/unit/support/shared/base_context.rb

@@ -12,6 +12,9 @@ shared_context "unit" do
     # Create a thing to store our temporary files so that they aren't
     # unlinked right away.
     @_temp_files = []
+
+    # Roughly simulate the embedded Bundler availability
+    $vagrant_bundler_runtime = Object.new
   end
 
   after(:each) do