Append to .out files for pre-emptable finalize job

Description

This should make it possible to see if a finalize job was pre-empted and requeued. By default, the requeued job overwrites the output file, removing the evidence.

How Has This Been Tested?

No easy way to test, other than deploying it and waiting to see results. The --open-mode=append option is taken from sbatch docs.

Types of changes

Checklist:

• My code follows the code style of this project.

Reviewers

@ahmedk @schmidtp

mentioned in merge request !564 (merged)

LGTM

merged

mentioned in commit 969cd856

changed milestone to %3.4.3