Depends-On: ansible/ansible-zuul-jobs#253
We cannot run the following tests without any ESXi host:
- vmware_drs_rule_info
- vmware_drs_rule_facts
- vmware_host_acceptance
- vmware_host_active_directory
- vmware_host_powerstate
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.
