Support graceful timeout for Sidekiq Cluster processes

Considering we're moving towards using Sidekiq Cluster
only and customers using Omnibus are allowed to configure
a Sidekiq timeout through -t flag, here we support a
-t flag for `bin/sidekiq-cluster`.

Here's how it works:

Internally, we'll pass the given value to Sidekiq (`-t`) flag.
For whatever value given, Sidekiq Cluster will wait for 5
extra seconds in order to hard stop all remaining alive processes.

So for instance, if `10` is given, Sidekiq will try to gracefully
terminate within this time. If it gets stuck for some reason, the
processes will be sigkilled within `15` seconds (total wait time).