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
