CRM-6736 drush civicrm install command should install in sites/all not sites/default

Right now drush civicrm-install determines the location of the modules directory using:

drush_get_context('DRUSH_DRUPAL_SITE_ROOT', FALSE);

That will return the directory with the Drupal settings.php file in it.

Therefor, drush will install civicrm either in the default directory or a directory named after the domain of the site.

civicrm, however, seems to want to be installed in sites/all/modules (as I learned from civicrm.settings.php - which sometimes fails if civicrm is in the default directory).

I've attached a patch that hard codes this behavior. I'm not convinced that's the right way to do it either... but am a bit stumped. Or - should I submit a patch for civicrm.settings.php ?