[doc] Add previously undocumented options in Terraform/Openstack (#11477)

These options are available in https://github.com/zonca/jetstream_kubespray/blob/6ba36564107c34a6261f2ae44d05aa344277e10e/contrib/terraform/openstack/modules/compute/variables.tf#L204-L218 but are currently undocumented in the README.md