ci: Add OOD image build job to gitlab CI

# Conflicts:
#   openstack-ood/variables.pkr.hcl

We do not run merge request pipeline, so no need to check for it

Download group_vars/knightly or group_vars/prod for knightly or prod
environments respectively.
Note: For all other environments like dev or staging just use the
default values from group_vars/all

We moved cheaha.yml playbook to run during deploy time

to avoid error when a value is not provided for root_ssh_key