Set default_value_for PxeServer#customization_directory

https://bugzilla.redhat.com/show_bug.cgi?id=1081006
Migrate any PXE server #customization_directory that is nil to ""

https://bugzilla.redhat.com/show_bug.cgi?id=1081006
Merge pull request #209 from brandondunne/validate_pxe_server_customization_directory

Validate that PXE servers have a customization_directory