Update GDK doc for enable puma by default

This is the document update due to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30285.

There are two work contained in this MR:

1. Add instruction how to enable Unicorn.

• Update doc/howto/README.md
• Add a new file doc/howto/unicorn.md

2. Update existing documents related to web application server(it was 'unicorn'). I search 'unicorn' in GDK project, I found 5 files may need to update.

• File 1, doc/howto/troubleshooting.md
• File 2, doc/howto/configuration.md
• File 3, doc/howto/pry.md
• File 4, support/templates/gitlab.yml.erb
• File 5, lib/gdk.rb