When using ironic virt driver, if scheduled_node is not specified
in rebuild_instance compute manager method (as it happens in case
of instance rebuild), the first ironic node is selected:
computes = ComputeNodeList.get_all_by_host(context, host, use_slave)
return computes[0]
After the first rebuild, instance.node is updated to be this first
ironic node, which causes subsequent rebuilds to fail, as virt driver
tries to set instance_uuid on a newly selected ironic node and fails.
Closes-bug: #1564921
Change-Id: I2fe6e439135ba6aa4120735d030ced31081ef202
(cherry picked from commit 54b122caec)