private DeploymentSpec.UpgradeRollout readUpgradeRollout(String rollout) {
        return switch (rollout) {
            case "separate" -> UpgradeRollout.separate;
            case "leading" -> UpgradeRollout.leading;
            case "simultaneous" -> UpgradeRollout.simultaneous;
            default -> throw new IllegalArgumentException("Illegal upgrade rollout '" + rollout + "': " +
                                                          "Must be one of 'separate', 'leading', 'simultaneous'");
        };
    }
Returns true if the given value is "true", or if it is missing
config-model-api/src/main/java/com/yahoo/config/application/api/xml/DeploymentSpecXmlReader.java