Prevent deletion of local cluster

[dharmit]

Issue:

rancher/rancher#32745
rancher/rancher#48303

Problem

Presently, it's possible to delete the local cluster using:

$ k delete cluster.management.cattle.io local

Other ways the local cluster, one on which Rancher is deployed, could be corrupted are mentioned in rancher/rancher#48303.

Solution

• Prevent local cluster deletion:

  $  k delete cluster.management.cattle.io local
  Error from server (BadRequest): admission webhook "rancher.cattle.io.clusters.management.cattle.io" denied the request: cannot delete the local cluster

• Prevent deletion of local and fleet-local namespaces:

  $ k delete ns local
  Error from server (BadRequest): admission webhook "rancher.cattle.io.namespaces" denied the request: deletion of namespace "local" is not allowed
  
  $ k delete ns fleet-local
  Error from server (BadRequest): admission webhook "rancher.cattle.io.namespaces" denied the request: deletion of namespace "fleet-local" is not allowed

CheckList

• Test
• Docs

[ericpromislow]

Given that "local" already appears in that file, I would set it to a constant called localCluster, like in pkg/resolvers/grbRuleResolvers.go

[jakefhyde]

LGTM, but just wanted to confirm that the agent's CLUSTER_CLEANUP is unimpacted.