Support "sequential" mode for executing load tests

[rschwietzke]

The command line argument sequential mode should become a property so that one can configure a load test setup to run one after the other user.

Limitations: One user per test case only, iteration based only, no load factor or profiles or other things will be obeyed. Rampup is ignored, shutdown and warmup are not. Measurement can be used to limit the entire length, but when set long enought, the test will finish by itself correctly.

Suggestion: A second version of the switch would be nice to support interleaving of the runs. So instead of running (assuming three test cases with each 3 iterations) 1/1/1/2/2/2/3/3/3, we run 1/2/3/1/2/31/2/3. When the number of iterations per run is not the same, it will be executed what is left such as 1/2/3/1/2/3/1/3 (2 is only to be executed 2 times).