Revert "Make $rabbitmq::params::package_apt_pin default to undef"

Author: Morgan Haskel

manifests/params.pp

@@ -66,7 +66,7 @@
   #install
   $admin_enable               = true
   $management_port            = '15672'
-  $package_apt_pin            = undef
+  $package_apt_pin            = ''
   $package_gpg_key            = 'http://www.rabbitmq.com/rabbitmq-signing-key-public.asc'
   $repos_ensure               = true
   $manage_repos               = undef