Two changes:
1. Move bin/web to bin/web_unicorn(removed PUMA switching logic)
Introduce new shadow script bin/web. Now we have 3 scripts working
together: bin/web, bin/web_puma, bin/web_unicorn

  In bin/web, it checks ENV['USE_WEB_SERVER']:
    - if value is 'puma' or value is not set, call bin/web_puma
    - if value is 'unicorn', call bin/web_unicorn
    - report error if other values given

2. Gitlab rails allow ENV setting to override timeout
    - ENV['GITLAB_RAILS_RACK_TIMEOUT'] for service_timeout
    - ENV['GITLAB_RAILS_WAIT_TIMEOUT'] for wait_timeout