devstack: Stop hacking blessed into global-requirements
devstack: Stop hacking blessed into global-requirements

Forgot this in the last devstack patch.

This stops having devstack add blessed to openstack global-requirements.

Also, this renames BUILD_AKANDA_DEV_APPLIANCE to the more intuitive
BUILD_AKANDA_APPLIANCE_IMAGE.

Change-Id: Ic69018e45d103a383daf17ef7c82b8f727554d21

openstack/akanda-rug,stackforge/akanda-rug,stackforge/akanda-rug,openstack/akanda-rug