test(inspec): enable use_upstream_repo for debian & centos-6

* Add new `centos-6` image
* Install `9.6` from upstream repo

test/integration/default/controls/services_spec.rb

@@ -1,8 +1,14 @@
+# Overide by OS
+service_name = 'postgresql'
+if os[:name] == 'centos' and os[:release].start_with?('6')
+  service_name = 'postgresql-9.6'
+end
+
 control 'Postgres service' do
   impact 0.5
   title 'should be running and enabled'
 
-  describe service('postgresql') do
+  describe service(service_name) do
     it { should be_enabled }
     it { should be_running }
   end