allow using gitlab-shell installation from tarballs and use load path instead of require_relative

1. Should not require git repo for gitlab-shell, I want to package it separately from tag tarballs.
2. By default we install lib/* to /usr/lib/vendor_ruby and it will be available from ruby load path. So instead of using require_relative, if you add lib directory to load path, both cases will work.

** Execute gitlab:shell:install
fatal: Not a git repository (or any of the parent directories): .git
fatal: Not a git repository (or any of the parent directories): .git
fatal: Not a git repository (or any of the parent directories): .git
bin/install:3:in `require_relative': cannot load such file -- /usr/share/gitlab
-shell/lib/gitlab_init (LoadError)
        from bin/install:3:in `<main>'
bin/create-hooks:7:in `require_relative': cannot load such file -- /usr/share/g
itlab-shell/lib/gitlab_init (LoadError)
        from bin/create-hooks:7:in `<main>'
rake aborted!
Errno::ENOENT: No such file or directory @ rb_sysopen - /usr/share/gitlab/.ssh/
environment
/usr/share/gitlab/lib/tasks/gitlab/shell.rake:73:in `initialize'
/usr/share/gitlab/lib/tasks/gitlab/shell.rake:73:in `open'
/usr/share/gitlab/lib/tasks/gitlab/shell.rake:73:in `block (3 levels) in <top (
required)>'
/usr/lib/ruby/vendor_ruby/rake/task.rb:240:in `call'
/usr/lib/ruby/vendor_ruby/rake/task.rb:240:in `block in execute'
/usr/lib/ruby/vendor_ruby/rake/task.rb:235:in `each'
/usr/lib/ruby/vendor_ruby/rake/task.rb:235:in `execute'
/usr/lib/ruby/vendor_ruby/rake/task.rb:179:in `block in invoke_with_call_chain'
/usr/lib/ruby/2.1.0/monitor.rb:211:in `mon_synchronize'
/usr/lib/ruby/vendor_ruby/rake/task.rb:172:in `invoke_with_call_chain'
/usr/lib/ruby/vendor_ruby/rake/task.rb:165:in `invoke'
/usr/lib/ruby/vendor_ruby/rake/application.rb:150:in `invoke_task'
/usr/lib/ruby/vendor_ruby/rake/application.rb:106:in `block (2 levels) in top_level'
/usr/lib/ruby/vendor_ruby/rake/application.rb:106:in `each'
/usr/lib/ruby/vendor_ruby/rake/application.rb:106:in `block in top_level'
/usr/lib/ruby/vendor_ruby/rake/application.rb:115:in `run_with_threads'
/usr/lib/ruby/vendor_ruby/rake/application.rb:100:in `top_level'
/usr/lib/ruby/vendor_ruby/rake/application.rb:78:in `block in run'
/usr/lib/ruby/vendor_ruby/rake/application.rb:176:in `standard_exception_handling'
/usr/lib/ruby/vendor_ruby/rake/application.rb:75:in `run'
/usr/bin/rake:27:in `<main>'   
Tasks: TOP => gitlab:shell:install