fix foreman_rex_cockpit acceptance test

1. the family is called `redhat`, not `centos`
2. to apply a manifest, you need to define `manifest` not `pp`

Author: evgeni

spec/acceptance/foreman_rex_cockpit_spec.rb

@@ -1,11 +1,10 @@
 require 'spec_helper_acceptance'
 
-describe 'Scenario: install foreman with rex cockpit', if: os[:family] == 'centos' do
+describe 'Scenario: install foreman with rex cockpit', if: os[:family] == 'redhat' do
   before(:context) { purge_foreman }
 
-
   it_behaves_like 'an idempotent resource' do
-    let(:pp) do
+    let(:manifest) do
       <<-PUPPET
       include foreman
       include foreman::plugin::remote_execution::cockpit