title: Symfony2 Dependency Injection load configs based on a condition
body: <p>I want to load configuration parameters based on a condition in symfony2.</p>
  
  <p>If given "driver" parameter is "a" then require and load this parameters, else, require and load another parameters.</p>
  
  <p>In dependency injection configuration, i must set some rules to all parameters like can not be null etc. </p>
  
  <p>How can i do this?</p>