Fix system_metadata "None" and created_at values

The ill-fated migration 153 converted None instance_type values to
"None" in the database instead of properly making them NULL. This
corrects that by sweeping all of the likely instance_type_% values and
converting "None" to NULL. Also, it adds a belated created_at stamp
to all of the items, which was missing in 153 as well.

It also corrects the 153 migration to avoid polluting things in the
first place for systems that haven't rolled through that yet.

Fixes bug 1152546

Change-Id: I3585463ae15abfd534f2ff070e2974f3cb51d7e8
Merge "Fix system_metadata "None" and created_at values"