Ability to start dedicated Sidekiq processes

This adds the ability to start Sidekiq processes for specific queues. See https://gitlab.com/gitlab-org/gitlab-ee/issues/1282 for more information.

bin/sidekiq-cluster

+#!/usr/bin/env ruby
+
+require 'optparse'
+require 'thread'
+require_relative '../lib/gitlab/sidekiq_cluster'
+require_relative '../lib/gitlab/sidekiq_cluster/cli'
+
+Thread.abort_on_exception = true
+
+cli = Gitlab::SidekiqCluster::CLI.new
+
+begin
+  cli.run
+rescue Gitlab::SidekiqCluster::CLI::CommandError => error
+  abort error.message
+end