Add the `vmware/scenario/vcenter_only` alias to be able to identify the
vmware test roles that can be tested with just on vcenter instance.
Also, add a check to be sure we don't try to attach ESXi hosts if none
has been defined.
* Fixed format issues reported by pre-check tests
* Changes as per review comments. using single api_client between util class and module
* re-running the task to check the idempotency of module object
