public Optional<Duration> hostTTL(InstanceName instance, Environment environment, RegionName region) {
        return (environment.isManuallyDeployed() ? devSpec.hostTTL
                                                 : instance(instance).flatMap(spec -> spec.hostTTL(environment, Optional.of(region))))
                .or(this::hostTTL);
    }
Additional host time-to-live for this application. Requires a custom cloud account to be set.
This also applies only to zones with dynamic provisioning, and is then the time hosts are
allowed remain empty, before being deprovisioned. This is useful for applications which frequently
deploy to, e.g., test and staging zones, and want to avoid the delay of having to provision hosts.
config-model-api/src/main/java/com/yahoo/config/application/api/DeploymentSpec.java