Make tenant_usage fall back to instance_type_id

In order to expose historical usage information about instances that
were deleted at the time migration #153 was run, we need to fall back
to looking up type information by instance_type_id.

This patch does that (only for deleted instances) and replicates the
previous behavior of skipping instances that have an instance_type_id
that points to a non-existent type.

Fixes bug 1161022

Change-Id: I39d30c59d1711d8f2a61c7a5e2545b3952e55dbe
Merge "Make tenant_usage fall back to instance_type_id"