Multiple processes using systemd as the init_system

[jsantos]

According to sidekiq's official example systemd unit file:

# This file corresponds to a single Sidekiq process.  Add multiple copies
# to run multiple processes (sidekiq-1, sidekiq-2, etc).

So far so good. But when using capistrano-sidekiq, if I set the init_system to :systemd the service will be started/restarted/stopped with:

execute :systemctl, '--user', <action>, fetch(:service_unit_name)

service_unit_name here is a single value and right now I believe we're not able to set multiple of these.

One idea might be to use something similar to each_process_with_index, but in this specific case I guess we won't need the PID (I'm actually switching to systemd due to the upcoming deprecations, so trying to avoid PIDs at all cost).

Let me know your thoughts about this, I can work on a PR once we reach an agreement.