add dreprecated retain alias : interval

The interval directive is deprecated but still used in older config files.

From rsnapshot documentation :
> retain [name] [number]
>
> "name'' refers to the name of this backup level (e.g., hourly, daily, so also called the 'interval'). ``number'' is the > number of snapshots for this type of interval that will be retained. The value of ``name'' will be the command > passed to rsnapshot to perform this type of backup.
> 
> A deprecated alias for 'retain' is 'interval'.