Added another drush domain path override.  #1550

--domains-file lets you specify another location for the domains list.  This lets us use a writable persistent directory on our acquia installation.  Unfortunately another part of the script expects to find domains.list files in drupal_root/sites.  I couldn't figure if there was a way to get sibling sites of a multisite in drush, and even if I did there was no guarantee their aliases would be configured with a domains-file, so I opted to add another option to the drush command.  --domains-file-root lets you point to a non-drupal root location to search for domains.list files.
Merge pull request #2567 from openscholar/1550-drush_domain_qa

Added another drush domain path override.  #1550