add user group processes config entry

when I use this framework to test our site I find out that it's not convenient to specify processes. So we want a configuration way to specify the amount of processes of a user group. This pull request is a way to add a config entry to the config.cfg file. In this pull request we can do this:

    [user_group-1]
    threads = 3
    processes = 4
    script = example_mock.py

the `processes` specifies that it has four processes for user_group-1 and each process has three threads to run. it  equals to this:

    [user_group-1] 
    threads = 3
    script = example_mock.py

    [user_group-2] 
    threads = 3
    script = example_mock.py

    [user_group-3] 
    threads = 3
    script = example_mock.py

    [user_group-4] 
    threads = 3
    script = example_mock.py

it's so tedious to do the duplicate works.

the `processes` entry is optional, if not specified, default value is 1 

thx