Set default authorized_keys_file

This is the same as gitlab-shell's default. This is to ensure
that it's always set.

It needs to be the same as gitlab-shell's default because we
don't set a default value in omnibus-gitlab. If users don't
set the value of that config in their install and they upgraded,
we must ensure that it's still going to point to the same
authorized keys file.

config/initializers/1_settings.rb

@@ -515,7 +515,7 @@ Settings['sidekiq']['log_format'] ||= 'default'
 Settings['gitlab_shell'] ||= Settingslogic.new({})
 Settings.gitlab_shell['path']           = Settings.absolute(Settings.gitlab_shell['path'] || Settings.gitlab['user_home'] + '/gitlab-shell/')
 Settings.gitlab_shell['hooks_path']     = :deprecated_use_gitlab_shell_path_instead
-Settings.gitlab_shell['authorized_keys_file'] ||= nil
+Settings.gitlab_shell['authorized_keys_file'] ||= File.join(Dir.home, '.ssh', 'authorized_keys')
 Settings.gitlab_shell['secret_file'] ||= Rails.root.join('.gitlab_shell_secret')
 Settings.gitlab_shell['receive_pack']   = true if Settings.gitlab_shell['receive_pack'].nil?
 Settings.gitlab_shell['upload_pack']    = true if Settings.gitlab_shell['upload_pack'].nil?