Restrict deleting credential store keys ones assigned to a resource

Once a key is generated and assigned to  compute resource can go and delete it from credential store page. Not even a warning message is prompted.

1. If keys are assigned, to a compute resource or a storage resource, should not be able to delete. Either we disable the delete button OR prompt with a meaningful message. 
2. Even when keys are not assigned, provide a warning message and get user to confirm the deletion. Something like 'Do you really want to delete this key?"