Added support for importing keys via primary/security mirror clauses

[Vultaire]

Proposed Commit Message

Added support for importing keys via primary/security mirror clauses

Added support for importing keys via primary/security mirror clauses

Presently, mirror keys cannot be associated with primary/security
mirrors.  Unfortunately, this prevents use of Landscape-managed
package mirrors as the mirror key for the Landscape-hosted repository
cannot be provided.

This patch allows the same key-related fields usable on "sources"
entries to be used on the "primary" and "security" entries as well.

LP: #1925395

Test Steps

Note: These steps require a running Landscape server.

• Following the Landscape repository documentation, set up a mirror key, create an ubuntu distribution with default pockets, and sync the pockets. Run "landscape-api get-distributions" and grep for "apt" to find the mirror line to use.
• Via cloud-init userdata, add the mirror to the apt.packages and/or apt.security sections, including the public key via the key property, and use this to spawn a VM or container.
• Without the fix, the mirrors will be added but "apt update" will fail due to the repository's key not being recognized. With the fix, it should work.

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

[Vultaire]

Please note: my initial commit to tools/.github-cla-signers (#881) is still pending.

[github-actions]

Hello! Thank you for this proposed change to cloud-init. This pull request is now marked as stale as it has not seen any activity in 14 days. If no activity occurs within the next 7 days, this pull request will automatically close.

If you are waiting for code review and you are seeing this message, apologies! Please reply, tagging mitechie, and he will ensure that someone takes a look soon.

(If the pull request is closed and you would like to continue working on it, please do tag mitechie to reopen it.)

[TheRealFalcon]

Thanks!