dummy mode: use execution time limit for sleep time

[oliver-sanders]

For dummy mode it may be useful to have tasks sleep for a percentage of their execution time limit to achieve more realistic simulation. This would probably be an optional configuration.

[hjoliver]

Very early versions of Cylc actually had proportional dummy (or sim mode, maybe?) run length. Dropped it because users had to configure timings that had no other use at the time. But now execution time limit is useful...