fix: install lxd snap only when snap list lxd reports absent

[blackboxsw]

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Proposed Commit Message

    fix: install lxd snap only when snap list lxd reports absent
    
    Enable cloud-init to 'snap install lxd' when 'snap list lxd' reports
    that the lxd snap is not available. Do not rely on which('lxd') because
    lxd-installer deb package delivers this script, not the lxd snap.
    
    Prior to Ubuntu Plucky, lxd-installer /usr/sbin/lxd would automatically
    install lxd snap if it wasn't on the system. In Ubuntu Plucky and newer
    lxd-installer now blocks on a user prompt "Would you like to install
    LXD.. ?" which prevents cloud-init from automatically installing LXD
    snap.
    
    LP: #2136198

Additional Context

Test Steps

CLOUD_INIT_KEEP_INSTANCE=1 CLOUD_INIT_OS_IMAGE=plucky CLOUD_INIT_CLOUD_INIT_SOURCE=IN_PLACE .tox/integration-tests/bin/pytest  tests/integration_tests/modules/test_lxd.py::test_basic_preseed

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>)

[holmanb]

The proposed commit message is too long, please wrap it.

Has the integration test been failing on plucky since it was released?

[holmanb]

How does backwards compatibility for this look on pre-plucky?

[blackboxsw]

Good and same behavior from snap list:

root@jj2:~# which lxd
root@jj2:~# snap list lxd
error: no matching snaps installed
root@jj2:~# echo $?
1

---- noble
root@nn:~# which lxd
/usr/sbin/lxd
root@nn:~# snap list lxd
error: no matching snaps installed
root@nn:~# echo $?
1

[holmanb]

Thanks @blackboxsw