CUDAEnsemble log files always append

[Robadob]

Noticed by Kenneth, if you run an ensemble with logging twice in a row all the files will be appended, rather than truncated.

The fix for step logs, should be rather simple change the last parameter here to true, and step log files will always truncate when logged via CUDAEnsemble.

Exit log files are a bit more complicated, as they are intended to append. So it would be necessary to delete any exit log files at the expected locations before outputting any logs.

There's probably some discussion to be had, over whether it would make more sense to purge the entire output dir before logging, to remove any confusion if the two runs create different files.

[Robadob]

Give user an option to control truncate or exception.