Avoid Yum/RPM prefetch in norpm provider

When package installation is disabled we still prefetch packages.
This disables the package prefetch by returning an empty array
which should be fine in the normal case and fixes issues when
running puppet in some docker containers.

Change-Id: Ia483c5f8500b804ba37a80e9ca1ec9c038f0a867

lib/puppet/provider/package/norpm.rb

@@ -33,4 +33,8 @@ Puppet::Type.type(:package).provide :norpm, :source => :rpm, :parent => :rpm do
     true
   end
 
+  def self.instances
+    return []
+  end
+
 end