Fixes #38009 - content-override with deselect

[m-bucher]

For host repository-sets with structured apt enabled setting content-overrides using bulk-action:
de-selecting deb-repos after using select-all has no effect.

What are the changes introduced in this pull request?

find_content_overrides() helper method must be adapted to work with one content for each katello-repo (#11058).

Considerations taken when implementing this change?

What are the testing steps for this pull request?

1. Enable structured apt and have multiple structured deb-content
2. Create Host attached to deb-content
3. Try to reset content-overrides for that host in the following way:
4. Select all content
5. De-select part of the deb-repos
6. Use bulk-action to override content state

ToDo

• test Show all selected
• write more unit tests 🙈