Avoid processing fact yaml files with empty 'values' hash

Under certain conditions it can happen that Puppet Server creates a fact
yaml file for a node that contains an empty `values` hash, for example
when multiple Puppet Servers are accessed through a load balancer like
HAProxy where Puppet agent requests are distributed equally and randomly.

When trying to process such a facts file with an empty `values` hash
Foreman will reject the HTTP request with an error message like this:

    During the fact upload the server responded with: 422 Unprocessable Entity. Error is ignored and the execution continues.
    {"message":"ERF42-3829 [Foreman::Exception]: invalid facts, missing operating system value"}

This change adds a simple method that loads the fact yaml file and
returns true if the `values` hash is empty. Such fact yaml files are
then skipped when processing host facts and a warning is written to
STDOUT. The Puppet administrator can then examine and delete any invalid
fact yaml files.

files/external_node_v2.rb

@@ -93,9 +93,21 @@ class FactUploadError < StandardError; end
   end
 end
 
+def empty_values_hash?(facts_file)
+  facts = File.read(facts_file)
+  puppet_facts = YAML::load(facts.gsub(/\!ruby\/object.*$/,''))
+
+  puppet_facts['values'].empty?
+end
+
 def process_host_facts(certname)
     f = "#{puppetdir}/yaml/facts/#{certname}.yaml"
     if File.size(f) != 0
+      if empty_values_hash?(f)
+        puts "Empty values hash in fact file #{f}, not uploading"
+        return 0
+      end
+
       req = generate_fact_request(certname, f)
       begin
         upload_facts(certname, req) if req
@@ -115,6 +127,10 @@ def process_all_facts(http_requests)
     certname = File.basename(f, ".yaml")
     # Skip empty host fact yaml files
     if File.size(f) != 0
+      if empty_values_hash?(f)
+        puts "Empty values hash in fact file #{f}, not uploading"
+        next
+      end
       req = generate_fact_request(certname, f)
       if http_requests
         http_requests << [certname, req]