Fix build-image.sh example comments
Fix build-image.sh example comments

Environment variables used to control settings in elements need to be
exported so that they are available to the script's subprocesses, so fix
the documented examples so they can easily be un-commented and used.

Change-Id: Ie003f98162afbe8ed827d6aba78e443323d92655

openstack-infra/project-config,openstack-infra/project-config