test(apt): add plucky version for hello pkg

[aciba90]

Proposed Commit Message

test(apt): add plucky version for hello pkg

Additional Context

https://jenkins.canonical.com/server-team/view/cloud-init/job/cloud-init-integration-plucky-lxd_container-minimal/4/testReport/junit/tests.integration_tests.modules/test_package_update_upgrade_install/test_versioned_packages_are_installed/

Test Steps

export CLOUD_INIT_PLATFORM=lxd_container
export CLOUD_INIT_OS_IMAGE=plucky

tox -e integration-tests -- tests/integration_tests/modules/test_package_update_upgrade_install.py::test_versioned_packages_are_installed

Merge type

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