Pass instance to _reschedule rather than instance_uuid

This is needed so that we can refactor _instance_update and pass the
instance object through to _instance_update and avoid out-of-band trips
to conductor to update an instance object and then have to refresh the
object elsewhere to get database updates.

Part of blueprint compute-manager-objects-juno

Change-Id: Ie9fcf16a7f7c32613e3f01dccd91da190c442ea7
Merge "Pass instance to _reschedule rather than instance_uuid"