Add and backfill project_id for p_ci_pipeline_variables

Ref: #463244 (closed)

What does this MR do and why?

This MR adds and backfills project_id for p_ci_pipeline_variables as this is a requirement for Cells1.0.

We have about ~2B records to update which should take ~6 weeks to complete on gitlab.com.

app/models/ci/pipeline_variable.rb

@@ -21,9 +21,16 @@ class PipelineVariable < Ci::ApplicationRecord
     alias_attribute :secret_value, :value
 
     validates :key, :pipeline, presence: true
+    before_validation :set_project_id, on: :create
 
     def hook_attrs
       { key: key, value: value }
     end
+
+    private
+
+    def set_project_id
+      self.project_id ||= pipeline&.project_id
+    end
   end
 end