Addresses #680 (comment)

This PR breaks the existing dependency for openondemand app installs to require a cluster to be deployed in the environment used for image builds.

• openondemand_*_partition is overridden for builder vars so that ood app install always included in image builds
• In this case the named partitions don't actually exist and so vars are simply truthy
• In the common environment, logic handles cases where openhpc_partitions is defined and populated, defined and empty and undefined