By using a `rescue` block, we can potentially hide a really problem in the
block and return a success. This is a bit problematic for a functional
test.
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.
