traffic_replay: Split out random group membership generation logic

traffic_replay: Split out random group membership generation logic

This doesn't change functionality at all. It just moves the probability
calculations out into separate functions.

We want to tweak the logic/implementation behind this code, but the
rest of assign_groups() doesn't really care how the underlying
probabilities are worked out, so long as it gets a suitably random
user/group membership each time round the loop.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>