Fix @persistent@ substitution logic.

Just handle @persistent@ as a normal boolean.
On 11/06/2014 07:38 AM, Alexandre Detiste wrote:
>> @@ -28,8 +28,7 @@ def parse_crontab(filename, withuser=True, monotonic=False):
>>      environment = { }
>>      random_delay = 1
>>      start_hours_range = '0'
>> -    persistent = False
>> -    persistent = True # '@persistent@' this line can be removed by sed depending on build options
>> +    persistent = True
> 
> why not just `persistent = @persistent@` here ?

I'd rather have parse_crontab just return the /wanted/ state of
persistent instead of switching it on/off. It makes for clearer code in
subsequent patches.