Add polymorphic relationship to container_node

This polymorphic association will allow container nodes to "live on" an
arbitrary entity belonging to another provider.

The purpose for this association is to capture the idea that Kubernetes
Container Nodes are actually running inside a VM or Physical host owned by
another provider.

https://trello.com/c/4wkBDSS6/128-5-containernode-cross-provider-associations

vmdb/app/models/container_node.rb

@@ -1,6 +1,7 @@
 class ContainerNode < ActiveRecord::Base
   include ReportableMixin
   # :name, :uid, :creation_timestamp, :resource_version
-  belongs_to  :ext_management_system, :foreign_key => "ems_id"
-  has_many :container_groups
+  belongs_to :ext_management_system, :foreign_key => "ems_id"
+  has_many   :container_groups
+  belongs_to :lives_on, :polymorphic => true
 end

vmdb/db/migrate/20150402133252_add_container_node_cross_provider_association.rb

@@ -0,0 +1,11 @@
+class AddContainerNodeCrossProviderAssociation < ActiveRecord::Migration
+  def up
+    add_column :container_nodes, :lives_on_type, :string
+    add_column :container_nodes, :lives_on_id,   :bigint
+  end
+
+  def down
+    remove_column :container_nodes, :lives_on_type
+    remove_column :container_nodes, :lives_on_id
+  end
+end