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.
Refactoring of the following roles to make use of the new
`prepare_vmware_tests` role.
- `vmware_resource_pool`
- `vmware_resource_pool_facts`
- `vmware_target_canonical_facts`
- `vmware_vcenter_settings`
- `vmware_vcenter_statistics`
- `vmware_vm_facts`
- `vmware_vm_host_drs_rule`
- `vmware_vm_vm_drs_rule`
- `vmware_vmkernel`
- `vmware_vmkernel_facts`
- `vmware_vspan_session`
- `vmware_vswitch`
- `vmware_vswitch_facts`
This patch depends on: https://github.com/ansible/ansible/pull/55719
Original PR: https://github.com/ansible/ansible/pull/54882
