Use authorized_keys lock when add new key to file

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

 v1.9.5
   - Put authorized_keys.lock in the same directory as authorized_keys
+  - Use lock file when add new entries to authorized_keys
 
 v1.9.4
   - Use lock file when modify authorized_keys

lib/gitlab_keys.rb

@@ -29,9 +29,11 @@ class GitlabKeys
   protected
 
   def add_key
+    lock do
       $logger.info "Adding key #{@key_id} => #{@key.inspect}"
       auth_line = key_line(@key_id, @key)
       open(auth_file, 'a') { |file| file.puts(auth_line) }
+    end
     true
   end