Force git ignore of drushrc.php and sites.php files.

If you add sites/all/drush/drushrc.php, sites/sites.php, or the right sites/env.domain.com/settings.php to your git repo, you will have problems pulling because aegir generates these automatically.

We should force add these files to a project's .gitignore, or at least provide a loud warning to users if these files are present in the git repo.