Ensure OOD app installs work for site image builds

[bertiethorpe]

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

[sjpb]

LGTM but needs CI running

[sjpb]

Image build bumped @ e8c1a39 appears to be https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/19185734179.

[sjpb]

There also appears to be a later image build run at e306e16 which failed: 601

Is there a reason we don't need that?

[bertiethorpe]

There also appears to be a later image build run at e306e16 which failed: 601

Is there a reason we don't need that?

That failed build was for SMS, testing purposes