[enhancement]: Add integration test for maas/curtin provisioning preseed interaction for cloud-init.postinst

# Enhancement
Since MAAS (via curtin boothooks) commissioning stage uses debconf-set-selections to trigger cloud-init.postinst to write supplemental preseed files to /etc/cloud/cloud.cfg.d prior to cloud-init first boot, we need an integration test asserting MAAS preseed behavior is not dropped or regressed over time. This can simply be done directly via debconf-set-selections/dpkg-reconfigure on a launched instance and validation of the rendered /etc/cloud/cloud.cfg.d files.

Long term, we may want a full integration test with a known MAAS dev server/service where we can test error-free commissioning of daily cloud-init images, but that is more likely an epic involving setting up a shared long-lived maas dev service and not a simple integration-test enhancement for cloud-init's daily integration tests.