Client concurrency scaling

[mmgaggle]

It would be great to have the ability to specify the clients, eg:

clients: [client1, client2, client3, client4 client5, client6]

Then also specify an array with the number of clients to use for each iteration, eg:

num_clients: [1, 2, 4, 6]

This would be useful for finding the client concurrency contention threshold for a cluster.