Add --run-seconds arg to ssbench-master run-scenario

...which trumps --op-count and stops generating benchmark jobs after the
specified number of seconds.

Like "operation_count", "run_seconds" may be specified in a scenario
file or on the command-line.  Any value for --op-count or --run-seconds
on the command-line trumps any value in the scenario file.  Also, the
auto-generated results filename is now of the form:

  .../<scenario_name>.u<user_count>.o<op_count>.r<run_seconds>.<timestamp>.stat

fixes #56