bugfix: disallow users from creating a vCluster in the platform namespace

[neogopher]

What issue type does this pull request address? (keep at least one, remove the others)
/kind bugfix

What does this pull request do? Which issues does it resolve? (use resolves #<issue_number> if possible)
resolves ENG-5603

Please provide a short message that should be published in the vcluster release notes
Fixed an issue where vcluster was allowed to be installed in the platform namespace

What else do we need to know?

Test Results:

Helm

1. When the platform installation exists, and the helm command is targeted towards the same namespace, it throws an error and exits.
   
2. When the helm install is targeted to a different namespace, it proceeds correctly.
   

vCluster CLI

1. When the platform installation exists, and the CLI command is targeted towards the same namespace, it throws an error and exits.
   
2. When the command is targeted towards a different namespace, it proceeds correctly.
   

[kale-amruta]

lgtm