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_drs_group`
- `vmware_drs_group_facts`
- `vmware_drs_rule_facts`
- `vmware_drs_portgroup`
- `vmware_drs_portgroup_facts`
- `vmware_dvs_portgroup_facts`
- `vmware_dvswitch`
- `vmware_dvswitch_pvlans`
This patch depends on: https://github.com/ansible/ansible/pull/55719
Original PR: https://github.com/ansible/ansible/pull/54882