The current approach requires catching exceptions to handle these errors
and callers are already handling model validations so it seems more
appropriate.  Also it seemed to convoluted to add this logic directly to
the model since the model needs to check too many possible associations
to determine whether or not there are more than one cluster since the
model doesn't know what it's being created on. Additionally we only
wanted to validate during create to avoid the risk of existing models
becoming invalid by many different edge cases.

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).

When this option is enabled, GitLab will create namespaces and service
accounts as usual. When disabled, GitLab wont create any project
specific kubernetes resources

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56557

- rewords examples starting with 'should'
- rewords examples starting with 'it'

Note: I had to manually fixup "onlies" to "only"

Domain was introduced on 11.8 and was not included on the
Project Cluster API. With this change user will be able to include
domain when adding and updating a cluster. Domain will also be included
on the GET calls.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59441

The flag is on by default, but allows us to revert back
to the old behaviour if we encounter any problems.

Modifies authorize! method to accept a third param, and then use it in
combination with 'add_cluster' policy to appropriately restrict adding
multiple clusters

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56110

Adds the following initial CRUD endpoints for Clusters API:

- GET list of clusters
- GET specific cluster
- POST add existing cluster (mimic of "Add cluster")
- PUT update cluser
- DELETE destroy cluster

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/40473