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.
In the VMware tests, we call the datastores `ds1` and `ds2`. The first
one is read-only, the second is read-write and can be used to deploy
VMs. The naming convention was not clear enough and source of a lot
confusion and mistake.
We now have two better names:
- ro_datastore, which is ... read-only
- rw_datastore, the one that we can use to deploy new VM.
* Fixed format issues reported by pre-check tests
* Changes as per review comments. using single api_client between util class and module
* re-running the task to check the idempotency of module object