Capistrano 2 fixes

[wingrunr21]

Hello!

I've got a few minor fixes for capistrano 2 that we encountered when trying to use this in our setup:

• There was an extraneous pty: false at the end of a run_as command
• sidekiq_user was not optional like it was in the capistrano 3 task
• The capistrano 3 task was using puma_user and not sidekiq_user
• Made sidekiq_user nil by default and added some documentation

Thanks!

[lacco]

It took a while for me figuring out the exact same issues addressed in this pull request. So please merge it, it's really great work! :)