tests: force version of cloud-init from PPA regardless of version

[blackboxsw]

Proposed Commit Message

    tests: force version of cloud-init from PPA regardless of version
    
    The PPA provided to CLOUD_INIT_CLOUD_INIT_SOURCE can contain a lower
    version of cloud-init than what is currently released for a given
    series. In those cases install an apt preferences file to pin
    the cloud-init installed to the given PPA regardless of the published
    version.

Additional Context

Test Steps

CLOUD_INIT_KEEP_INSTANCE=1 CLOUD_INIT_OS_IMAGE=jammy CLOUD_INIT_PLATFORM=lxd_vm CLOUD_INIT_CLOUD_INIT_SOURCE=ppa:cloud-init-dev/daily tox -e integration-tests -- tests/integration_tests/test_networking.py::test_invalid_network_v2_netplan

lxc exec cloudinit....... -- apt policy cloud-init  # ensure that the cloud-init version installed is 23.4.daily based instead of 24.1.3.

Checklist

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

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>)

[TheRealFalcon]

LGTM!