When Kubernetes clusters were originally built they could only
exist at the project level, and so there was logic included
that assumed there would only ever be a single Kubernetes
namespace per cluster. We now support clusters at the group
and instance level, which allows multiple namespaces.

This change consolidates various project-specific fallbacks to
generate namespaces, and hands all responsibility to the
Clusters::KubernetesNamespace model. There is now no concept of
a single namespace for a Clusters::Platforms::Kubernetes; to
retrieve a namespace a project must now be supplied in all cases.

This simplifies upcoming work to use a separate Kubernetes
namespace per project environment (instead of a namespace
per project).

Before this commit the wrong namespace could have been used in
Prometheus queries for group-level installations.