Fix broken unit tests because of list in notification drivers

Because the value for oslo_messaging_notifications/driver is now
a list[1], we should expect that a list is set when multiple drivers
are given.

[1] c7b0cc82fac79b47c3dd9a625cbd5a1eb192ed00

Change-Id: I2fbf613bdb32194e8862bc1eaaa0a7c76af01048

spec/classes/vitrage_init_spec.rb

@@ -88,7 +88,7 @@ describe 'vitrage' do
         before { params.merge!( :notification_driver => ['messaging', 'messagingv2']) }
 
         it { is_expected.to contain_vitrage_config('oslo_messaging_notifications/driver').with_value(
-          'messaging,messagingv2'
+          ['messaging', 'messagingv2']
         ) }
       end