Correct ovn-dbs upgrade image id variable
Correct ovn-dbs upgrade image id variable

In I8b5b25d03b86b2c44b2d47e5a0624e7dd13873da the variable
ovn_dbs_image_id was renamed to old_ovn_dbs_image_id but
one instance of it was left behind. This patch corrects
that.

Related-Bug: #1838971
Closes: rhbz#1758578
Change-Id: I5735210aad47411fdeb62d4dab45b1150ec3ea04

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates