Upgrade the database settings on a site during rb-site upgrade.

rb-site now transitions the settings_local.py files to use the new-style
DATABASES = {...} configuration. It attempts to import the keys we need
and then goes through the settings file, replacing the old keys with the
new ones.

New sites get the proper settings from the start.

Tested this with both creation and upgrading. They both work, and sites can be run from them.

One caveat, though. Right now, django-evolution breaks with the DATABASES = {...} keys. Mainly because the engine is assumed to be a short name ("sqlite3", etc.) instead of a full module path. There's a patch pending that fixes this. I'll be making sure that patch goes in and a release is done before this patch is in.