Add post-release checklist items to the PTL guide
Add post-release checklist items to the PTL guide

We forgot to add the database migration placeholders for Rocky:

  https://review.openstack.org/650964

So add the steps explicitly to the guide to help in the future.

Change-Id: If670a82cee59438f9bff9a962083ebf040b9a779

rahulunair/nova,openstack/nova,klmitch/nova,openstack/nova,klmitch/nova,mahak/nova,rahulunair/nova,klmitch/nova,mahak/nova,openstack/nova,mahak/nova,rahulunair/nova,klmitch/nova