public HostResource getHost(String hostAlias) {
        HostSpec hostSpec = provisioner.allocateHost(hostAlias);
        HostResource resource = hostname2host.get(hostSpec.hostname());
        return resource != null ? resource : addNewHost(hostSpec);
    }
The parent node for all Host instances, and thus accessible
to enable services to get their Host.

@author gjoranv
config-model/src/main/java/com/yahoo/vespa/model/HostSystem.java