Make 'file_path' a class method

This is required to use prefetch and also purge.

Change-Id: Ia7e10057637d26489bf4fa5589aaa1de6a03b151

lib/puppet/provider/barbican_config/openstackconfig.rb

@@ -20,7 +20,7 @@ Puppet::Type.type(:barbican_config).provide(
   :parent => Puppet::Type.type(:openstack_config).provider(:ruby)
 ) do
 
-  def file_path
+  def self.file_path
     '/etc/barbican/barbican.conf'
   end