Ensure overlay mounts create required directories

[Ndukiye]

Proposed Commit Message

fix(mounts): ensure overlay directories are created

Ensure required upper and work directories exist before overlay
mounts are applied. This prevents mount failures when directories
are missing during fstab processing.

Fixes #2916
LP: #1695277

## Additional Context
This change ensures cloud-init safely creates required directories
before attempting overlay filesystem mounts, improving robustness
on first boot scenarios.

## Test Steps
Run unit tests:
tox -e py3

Specifically:
pytest tests/unittests/config/test_cc_mounts.py -v

## Merge type

- [x] Squash merge using "Proposed Commit Message"
- [ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)