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

rabbitmq::repo::apt tests whether this value is true,
which fails with the future parser if default is the
empty string.

Author: raphink

manifests/params.pp

@@ -40,7 +40,7 @@
   #install
   $admin_enable               = true
   $management_port            = '15672'
-  $package_apt_pin            = ''
+  $package_apt_pin            = undef
   $package_gpg_key            = 'http://www.rabbitmq.com/rabbitmq-signing-key-public.asc'
   $repos_ensure               = true
   $manage_repos               = undef