fix: Update default LXD meta-data with user meta-data

[holmanb]

Proposed Commit Message

fix: Update default LXD meta-data with user meta-data

This was previously unnecessary because:

a. LXD automatically appends the user.meta-data key to default meta-data.
b. In the presence of duplicate keys, PyYAML uses the last key.

This change is part of a series of changes that will enable cloud-init to
avoid depending on undefined behavior. In the future LXD may stop appending
user-defined meta-data to its default meta-data. This change makes
cloud-init forward compatible to LXD for when that change is implemented.

GH-5575

Additional context

Fixes #5575.
Should make cloud-init forward compatible with LXD changes which might be made to address canonical/lxd#13853

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!