As downgrade are not supported after Kilo, we should remove them now.
Roll backs can be performed as mentioned in the below link:
http://docs.openstack.org/ops-guide/ops-upgrades.html#rolling-back-a-failed-upgrade
The DB downgrades were deprecated in Glance Mitaka release by commit
e3366afdfb227794fd46cfd8b2cf553a7ff1a709.
Change-Id: I937d15d93f16a3e44a50e6ff1a469098eab67c79
Implements: blueprint remove-db-downgrade