implemented ability to split sidekiq_roles by count of sidekiq-processes

Now we can customize how many processes capistrano has to run on each server.

set :sidekiq_role, [:sidekiq, :sidekiq_small, :sidekiq_big]

set :sidekiq_small_processes, 2
set :sidekiq_big_processes, 4

role :sidekiq, 'sidekiq.example.com'
role :sidekiq_small, 'sidekiq_small.example.com'
role :sidekiq_big, 'sidekiq_big.example.com'

It was very helpfull for me because we use different AWS-S3 instancess for sidekiq. I hope it will be helpfull for other developers.