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.
Refactor vmware_cluster into several modules (vmware_cluster, vmware_cluster_drs, vmware_cluster_ha and vmware_cluster_vsan) as discussed in #58023.
vmware_cluster lacks a lot of configuration options for DRS, HA and vSAN. Implementing them
all in vmware_cluster would make the module hard to maintain. Therefore, splitting it into several
modules and implementing the missing configuration options in them seems a good idea to me.
This is step one, refactoring vmware_cluster into several modules. Step two, implementing more
configuration options for DRS, HA and vSAN, will follow.
