@Override
        public Object filter(final Object var, final JinjavaInterpreter interpreter, final String... args) {
            final boolean useEnvVarSubst = (boolean) interpreter.getContext().get(USE_VARIABLE_SUBSTITUTION_KEY);
            if (useEnvVarSubst) {
                String envVarName = args[0];
                return "${" + envVarName + ":-" + var + "}";
            } else {
                return var;
            }
        }
@param var         The value being piped into the filter
@param interpreter
@param args        The filter args. One argument, the name of the env var to use for
                   substitution.
@return The value as a bash style variable substitution
stroom-app/src/test/java/stroom/dist/GenerateDistributionConfig.java