Fix apt pins generation

In Puppet 4 null value in
yaml backend is treated as Undef symbol. This
behaviour causes generate_apt_pins() function
to create pins with zero priority.

Closes-Bug: #1655961

Change-Id: I1d2bf737e88bba37e08e19c0be1f86c74f1dbd5c
(cherry picked from commit 4e37655b4e)

deployment/puppet/osnailyfacter/lib/puppet/parser/functions/generate_apt_pins.rb

@@ -19,7 +19,7 @@ and Codename. Repositories with no or empty priority are skipped.
 
     result = {}
     repositories.each do |repo|
-      next unless repo['priority']
+      next unless repo['priority'] and repo['priority'] != :undef
       uri = URI.parse "#{repo['uri']}/dists/#{repo['suite']}/Release"
       response = nil
       retry_count = 3