Make it possible to reuse existing Kubernetes cluster

Description

When someone wants to migrate to a new GitLab instance, existing Kubernetes cluster is not usable anymore. At this moment it is not possible to reuse existing Kubernetes cluster with gitlab-managed-apps installed there.

Theoretically it should be possible to propagate GitLab's database with data about apps installed in an existing cluster, but it appears it is not possible right now because we store generated Helm keys / certificates in the database.

Do you think it is something we should care about @mayra-cabrera @DylanGriffith?

Other options

For now, if we want to share a cluster with many projects, the best option is to use a group cluster integration or instance level cluster