Set site_dir to fix config imports on ACSF.

[danepowell]

BLT sets the $site_dir variable in blt.settings.php and uses this variable in three places:

1. Elsewhere in blt.settings.php (but only for ACE and local environments) to find settings include files.
2. In filesystem.settings.php (again, only for ACE environments)
3. In config.settings.php to determine where to look for config files (i.e. /config/$site_dir, usually /config/default)

The problem is that on ACSF, the site_dir is something like g/files/abc1234, because ACSF doesn't use true multisite. This is problematic because it prevents you from using a common configuration directory like config/default.

This patch overrides this behavior to set the site_dir to default on ACSF.

I could also see an argument for making the site_dir 'g', but this would make codebases less portable between ACE and ACSF (since you'd need to use a config directory like /config/default on ACE and /config/g on ACSF).

The only possible adverse effect I can see from this change is for sites that have legitimately separate configuration directories for all of their ACSF sites. But that just doesn't seem like it would be practical.