feat: Support templating the namespace in manifests

[sbernauer]

Description

Fixes #371
Also a bit of clean-up from #373

After this feature we should actually migrate demos to don't hard-code default.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author
- [ ] Changes are OpenShift compatible
- [ ] CRD changes approved
- [ ] Helm chart can be installed and deployed operator works
- [ ] Integration tests passed (for non trivial changes)

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added

[NickLarsenNZ]

LGTM

[Techassi]

One more note: Please make sure that the pre-commit checks pass.

[NickLarsenNZ]

LGTM