Use flavor in confirm-resize to drop claim

Current the confirm_resize() clean the stashed instance_type information
before invoking the drop_resize_claim, and the drop_resize_claim() try to
fetch the old instance_type in system metadata, which will be wrong.

We change the _cleanup_stored_instance_types() to pass back the
to_be_dropped instance_type, and use that information for drop_resize_claim.

Change-Id: Iba36acc75d3840104746596977b33bcd4e010f2e
Merge "Use flavor in confirm-resize to drop claim"