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.
The use of the `hostvars[esxi1].ansible_host` was attempt to use
different hostname and IP address. But it's actually the source of
more problems.
VMware expects to be able to resolvable the host name. This means, that
if someone wants to run the test-suite, s/he needs to use a DNS or
update the `/etc/hosts` files on the different hosts.
