Add parent physical chassis id to physical chassis

This additional field is used to model chassis nesting, for example
rack containing multiple enclosures that in turn contain multiple
sleds, containing one or more physical servers.

db/migrate/20180828092111_add_parent_physical_chassis_id_to_physical_chassis.rb

@@ -0,0 +1,5 @@
+class AddParentPhysicalChassisIdToPhysicalChassis < ActiveRecord::Migration[5.0]
+  def change
+    add_column :physical_chassis, :parent_physical_chassis_id, :bigint
+  end
+end