Updated settings for devstack django debug toolbar

#2177 Updated the Django debug toolbar and dev settings, but not devstack settings.  This caused `rake devstack` to raise an exception:

    django.core.exceptions.ImproperlyConfigured: Error importing debug panel debug_toolbar.panels.settings_vars: "No module named settings_vars"

This PR resolves the issue by updating devstack settings to match dev settings.

@flowerhack