Until now, the vcenter provider was switching between `static` and
`govcsim` depending on the presence of the following configuration file:
`test/integration/cloud-config-vcenter.ini`.
This was not consistent with Worldstream, which we enable with the
`VMWARE_TEST_PLATFORM` environment variable.
We now only rely on `VMWARE_TEST_PLATFORM` to know which platform should be
used. `govcsim` is still the default, this to preserve the original
behaviour.
This commit also rename the following variables to be consistent with the rest
of the code base. It also ensures they are alway defined, even with `govcsim`:
- `VCENTER_HOSTNAME`
- `VCENTER_USERNAME`
- `VCENTER_PASSWORD`
