Update the acceptance:facts rake task for beaker 5

The `output` method is no longer available to the test case. Instead
access the output from the result that's yielded to the method.

acceptance/util/puppet_facts_output.rb

@@ -132,8 +132,8 @@ def filename_from_facts(facts)
   end
 
   agents.each do |agent|
-    on agent, facter('--show-legacy -p -j'), :acceptable_exit_codes => [0] do
-      facts = JSON.parse(stdout)
+    on(agent, facter('--show-legacy -p -j'), :acceptable_exit_codes => [0]) do |result|
+      facts = JSON.parse(result.stdout)
       facter_major_version = facts['facterversion']
       primary_network = facts['networking']['primary']