Add Subject to OpenSCAP results

Adding subject_id and subject_type fields to openscap_results table
to allow assigning OpenSCAPResult to OpenStack VMs and later also
other inventory.

Existing container_id association is not touched in this PR, but it could
be migrated to polymorphic a subject association in future.

db/migrate/20170717084208_add_subject_to_open_scap_result.rb

@@ -0,0 +1,6 @@
+class AddSubjectToOpenScapResult < ActiveRecord::Migration[5.0]
+  def change
+    add_column :openscap_results, :subject_id,   :bigint
+    add_column :openscap_results, :subject_type, :string
+  end
+end

db/schema.yml

@@ -5415,6 +5415,8 @@ openscap_results:
 - id
 - container_image_id
 - created_at
+- subject_id
+- subject_type
 openscap_rule_results:
 - id
 - openscap_result_id