Fix preload

[jreidinger]

Problem

Preload does not work.

• https://trello.com/c/JQHROPwx/5468-make-sure-libzypp-preload-can-work

Solution

With help from libzypp guys it was found that default for CommitPolicy is DownloadAsNeeded, but for preload it needs DonwloadInAdvance. So we need to explicitly set it in code.

Testing

• Added a new unit test
• Tested manually

Screenshots

If the fix affects the UI attach some screenshots here.

Documentation

Remember to look at it from the user's perspective. Yes you have made the compiler happy.
But will the humans even know about your contribution? Sometimes they cannot miss it,
other times they need advertisement and explanation.

Look for relevant sections and adjust:

• The description parts of the JSON schema
• Is the CLI affected? See cli.md for a complete overview,
  change the /// comments (rust doc)
  and update the .md with cargo xtask markdown
• https://agama-project.github.io/ (source)
• Run: git ls-files '*.md'