ppc64le: Adjust containers_base_image var scope
ppc64le: Adjust containers_base_image var scope

containers_base_image is read from the top level scope of the
playbook's vars. Initially I placed it in the zuul vars but this
was incorrect.

Change-Id: Idf264aa3702ebc3d3e90d0c044e573ceab9f433a

redhat-openstack/rdo-infra,rdo-infra/ci-config,rdo-infra/ci-config,rdo-infra/ci-config,redhat-openstack/rdo-infra,rdo-infra/ci-config,redhat-openstack/rdo-infra,redhat-openstack/rdo-infra