Create .gitlab_shell_secret

add this file to fix this bug 
lib/gitlab_net.rb:122:in `read': No such file or directory @ rb_sysopen - ./.gitlab_shell_secret (Errno::ENOENT)
	from lib/gitlab_net.rb:122:in `secret_token'
	from lib/gitlab_net.rb:79:in `get'
	from lib/gitlab_net.rb:34:in `discover'
	from lib/gitlab_shell.rb:82:in `user'
	from lib/gitlab_shell.rb:87:in `username'
	from lib/gitlab_shell.rb:36:in `exec'
	from bin/gitlab-shell:16:in `<main>'