Forbid replacing secret with new account for existing Shoots

[ialidzhikov]

What would you like to be added:
Currently we don't have a validation that would prevent user to replace its cloudprovider secret with credentials for another account. Basically we do have only a warning in the dashboard - ref gardener/dashboard#422.

Steps to reproduce:

1. Get an existing Shoot.
2. Update its secret with credentials for another account.
3. Ensure that on new reconciliation, new infra resources will be created in the new account. The old infra resources and machines in the old account will leak.
   For me the reconciliation failed at

    lastOperation:
      description: Waiting until the Kubernetes API server can connect to the Shoot
        workers
      lastUpdateTime: "2020-02-20T14:56:43Z"
      progress: 89
      state: Processing
      type: Reconcile

wtih reason

$ k describe svc -n kube-system vpn-shoot
Events:
  Type     Reason                   Age                  From                Message
  ----     ------                   ----                 ----                -------
  Normal   EnsuringLoadBalancer     7m38s (x6 over 10m)  service-controller  Ensuring load balancer
  Warning  SyncLoadBalancerFailed   7m37s (x6 over 10m)  service-controller  Error syncing load balancer: failed to ensure load balancer: could not find any suitable subnets for creating the ELB

Why is this needed:
Prevent users to harm themselves.