Spec fix for parameters

- Use it_behaves_like 'an idempotent resource' in parameter_spec
- Hacky spec test fix for lack of idempotency when installing implicitly
  enabled plugins with newer RabbitMQ versions (see voxpupuli#844)

spec/acceptance/parameter_spec.rb

@@ -17,7 +17,7 @@ class { 'rabbitmq':
         admin_enable      => true,
       }
 
-      rabbitmq_plugin { [ 'rabbitmq_federation_management', 'rabbitmq_federation' ]:
+      rabbitmq_plugin { [ 'rabbitmq_federation_management' ]:
         ensure => present
       } ~> Service['rabbitmq-server']
 
@@ -33,11 +33,10 @@ class { 'rabbitmq':
         },
       }
       EOS
-
-      apply_manifest(pp, catch_failures: true)
-      apply_manifest(pp, catch_changes: true)
     end
 
+    it_behaves_like 'an idempotent resource'
+
     it 'has the parameter' do
       shell('rabbitmqctl list_parameters -p fedhost') do |r|
         expect(r.stdout).to match(%r{federation-upstream.*documentumFed.*expires.*3600000})