private void allocateHost(AbstractService service, HostSystem hostSystem, Element producerSpec) {
            // TODO store service on something else than HostSystem, to not make that overloaded
            service.setHostResource(hostSystem.getHost(producerSpec.getAttribute("hostalias")));
        }
Allocates a host to the service using host file or create service spec for provisioner to use later
Pre-condition: producerSpec is non-null

@param service the service to allocate a host for
@param hostSystem a {@link HostSystem}
@param producerSpec xml element for the service
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/VespaDomBuilder.java