Method override for Automate Domains

If a similar named method  exists in a
high priority domain that will overlay the existing method.
The usecase for this is a user wants to modify a single method from
the ManageIQ domain to the Customer domain. During resolution the
method from higher priority Customer domain will be used.
Merge pull request #103 from mkanoor/overlay_methods

Method override for Automate Domains
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