Render DELETE action for notifications

Notifications can be deleted through the DELETE method, yet we don't
render that as an available action when we return notification members
through GET. This is beecause the method has been enabled but no
action has been configured. Configuring a delete action solves this
problem.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1420872

config/api.yml

@@ -1019,6 +1019,8 @@
       :post:
       - :name: mark_as_seen
       - :name: delete
+      :delete:
+      - :name: delete
   :orchestration_stacks:
     :description: Orchestration Stacks
     :options:

spec/requests/api/notifications_spec.rb

@@ -15,6 +15,23 @@
     end
   end
 
+  describe "notification read" do
+    it "renders the available actions" do
+      api_basic_authorize
+
+      run_get(notification_url)
+
+      expected = {
+        "actions" => a_collection_including(
+          a_hash_including("name" => "mark_as_seen", "method" => "post"),
+          a_hash_including("name" => "delete", "method" => "post"),
+          a_hash_including("name" => "delete", "method" => "delete")
+        )
+      }
+      expect(response.parsed_body).to include(expected)
+    end
+  end
+
   describe 'notification edit' do
     it 'is not supported' do
       api_basic_authorize