Resolve "Multiple clusters: incorrect cluster details injected - environment scope is ignored"

What does this MR do?

Resolves incorrect output of deployment_platform when the Project has multiple clusters and multiple environments

Why was this MR needed?

Same kubernetes_platform was being returned on deployment_platform method, completely ignoring the environment argument on EE. This was resolved by rearranging DeploymentPlatform methods in the following way:

• Change find_cluster_platform_kubernetes to search for a specific environment on EE, and ignore environment argument on CE
• Override find_cluster_platform_kubernetes and deployment_platform methods
• Remove memoization from deployment_platform on EE, otherwise when searching for another platform, once the first one is assigned, environment parameter is completely ignored

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5347 (closed)

CE Port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17904