Stack overcloud upgrade prepare fails during FFU from 13 to 16

Issue

• During the following step [1] of the fast-forward upgrade process, the stack update fails with status UPDATE_FAILED.
• You use a custom template directory instead of /usr/share/openstack-tripleo-heat-templates in your stack for the --templates variable.

• You may get an error that looks like the following:

  (resources[1]: '<' not supported between instances of 'datetime.datetime' and 'NoneType')
  

[1] https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.2/html-single/framework_for_upgrades_13_to_16.2/index#running-the-overcloud-upgrade-preparation-upgrading-overcloud-standard