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.
vmware: refactoring of vmware test roles -- part5
Signed-off-by: Abhijeet Kasurde <akasurde@redhat.com>