public static HostFilter from(Collection<String> hostNames,
                                  Collection<String> flavors,
                                  Collection<ClusterSpec.Type> clusterTypes,
                                  Collection<ClusterSpec.Id> clusterIds) {
        return new HostFilter(Set.copyOf(hostNames),
                              Set.copyOf(flavors),
                              Set.copyOf(clusterTypes),
                              Set.copyOf(clusterIds));
    }
Returns a host filter from three optional conditions
config-provisioning/src/main/java/com/yahoo/config/provision/HostFilter.java