Refactor code around scheduling cluster installations

What does this MR do?

Refactors code around scheduling cluster installations

Are there points in the code the reviewer needs to double check?

no

Why was this MR needed?

To simplify schedule service for the cluster applications. I believe it better if schedule_installation_service is about scheduling rather than creating application records. Benefits are next:

1. It simplifies the code a bit.
2. It allows creating applications in the controller which means we can pass arguments to it.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Extracted from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19019