for spawning of each class with <fixed_count> class field.
Fixed users spawn first. The weight parameter for them is ignored.
Also added unit-tests for cases with fixed users.

…atio,

now it is considered based on the real number of users.
On the Task tab (web ui) the data is updated every 1 second, so it is possible
to see the actual ratio changing.
For command-line arguments --show-task-ratio, --show-task-ratio-json,
the behavior is also changed - the ratio pre-calculation based on passed num_users.
If there is no fixed_count users and num_users argument is None,
the old behaviour occurs.

Because in case then UserDispatcher._users_on_workers
is not fills instantly (see UserDispatcher._distribute_users)
we cant to monitore actual count of each user.
Also added some additional tests, includes to check this behaviour.

…ation.