obs_factory: Export the staging workflow if there is one

So that tools digesting obs_factory's APIs know what to ignore

src/api/app/models/obs_factory/staging_project.rb

@@ -58,6 +58,11 @@ def workflow_present?
       project.staging_workflow.present?
     end
 
+    # @return [Integer] id of the staging work flow (or nil)
+    def staging_workflow
+      project.staging_workflow_id
+    end
+
     # Part of the name shared by all the staging projects belonging to the same
     # distribution
     #
@@ -203,7 +208,7 @@ def self.attributes
        'building_repositories', 'broken_packages',
        'untracked_requests', 'missing_reviews',
        'selected_requests', 'overall_state',
-       'checks', 'missing_checks']
+       'checks', 'missing_checks', 'staging_workflow']
     end
 
     # Required by ActiveModel::Serializers