feat(integration_test): add CLOUD_INIT_PKG setting

[aciba90]

Proposed Commit Message

feat(integration_test): add CLOUD_INIT_PKG setting
    
Add CLOUD_INIT_PKG to control what specific cloud-init package to
install when CLOUD_INIT_SOURCE is PROPOSED, UPGRADE or points to a ppa.
    
SC-1858

Additional Context

Test Steps

export CLOUD_INIT_PLATFORM=lxd_vm
export CLOUD_INIT_OS_IMAGE=oracular

CLOUD_INIT_CLOUD_INIT_SOURCE=ppa:aciba/us-120-cloud-specific-cloud-init-packaging \
CLOUD_INIT_CLOUD_INIT_PKG=cloud-init-smart-os \
	tox -e integration-tests -- -vvv --pdb tests/integration_tests/modules/test_set_hostname.py::TestHostname::test_hostname

export CLOUD_INIT_CLOUD_INIT_PKG=cloud-init

CLOUD_INIT_CLOUD_INIT_SOURCE=PROPOSED \
	tox -e integration-tests -- -vvv --pdb tests/integration_tests/modules/test_set_hostname.py::TestHostname::test_hostname

CLOUD_INIT_CLOUD_INIT_SOURCE=UPGRADE \
	tox -e integration-tests -- -vvv --pdb tests/integration_tests/modules/test_set_hostname.py::TestHostname::test_hostname

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>)

[blackboxsw]

LGTM. It gives us integration test flexibility to mandate the specific subset of cloud-init packages installed.

Not for this PR: we may at some point need to define a mapping of

• platform/distro/series TO default_cloud_init_pkg_name

Stable releases will likely not get this update, so we probably will eventually want a dict representing known defaults at some point to avoid accidentally running integration tests with a default setting on an Ubuntu devel series and not testing the virtual package we should be testing because the default is set to cloud-init instead of cloud-init-base or cloud-init-azure if we define cloud-specific packages.