Fix instance type cleanup when doing a same-id migration

Previously, we assumed that we were moving from one flavor to another during
a migration (i.e. a resize). That assumption failed when a regular migration
went to delete the old_ stashed instance types from system_metadata.

This unifies the cleanup logic, which only gets performed if we're actually
doing a resize.

Fixes bug 1136768

Change-Id: I0796864b044fc746583164e4a684f79dc431c137
Merge "Fix instance type cleanup when doing a same-id migration"