It is impossible to boot instance from flavor with ephemeral disk when use_cow_images is set to 0

Detailed bug description:
During the verification of #1589490 the following error was raised:

Steps to reproduce:

deploy cluster 9.1
  1 controller
  1 compute

check:
root@node-2:~# ll /var/lib/nova/instances/
total 0
drwxr-xr-x 2 nova nova 6 Aug 17 12:52 ./
drwxr-xr-x 10 nova nova 105 Aug 18 10:17 ../

on compute:
set use_cow_images=0 value in nova config
service nova-compute restart

create flavor:
  nova flavor-create --ephemeral 1 'test_fl' 'auto' 1024 5 1
boot instance:
  nova boot new --image [image_id] --nic net-id=[internal_net_id] --flavor [test_flavor_id]

check:
root@node-2:~# ll /var/lib/nova/instances/_base/
total 33080
drwxr-xr-x 2 nova nova 53 Aug 18 11:04 ./
drwxr-xr-x 4 nova nova 30 Aug 18 11:04 ../
-rw-r--r-- 1 nova nova 51397120 Aug 18 11:04 724905800dd5458c4bc6a213b009e2234e97868f

Expected results:
boot passed
Actual result:
boot failed
Reproducibility:
100%
Workaround:
boot any instance with use_cow_images=True before
Impact:
 <put your information here>
Description of the environment:
snapshot #151

Additional information:
nova-conductor error:
9-4004-bf7a-ff8d0c5582d6] return cctxt.call(ctxt, 'deallocate_for_instance', **kwargs)
2016-08-18 11:05:05.645 13310 ERROR nova.conductor.manager [instance: 85db4722-bdd9-4004-bf7a-ff8d0c5582d6] File "/usr/lib/pytho
2016-08-18 11:04:05 ERR nova.scheduler.utils [req-f6fdb0ac-32c3-4aac-a09d-dc78428f979a 3af9b67e1c9f45d99c28295a9584a074 11090423f8fb45b1b249b799ba34ec22 - - -] [instance: 85db4722-bdd9-4004-bf7a-ff8d0c5582d6] Error from last host: node-2.test.domain.local (node node-2.test.domain.local): [u'Traceback (most recent call last):\n', u' File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 1926, in _do_build_and_run_instance\n filter_properties)\n', u' File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 2116, in _build_and_run_instance\n instance_uuid=instance.uuid, reason=six.text_type(e))\n', u'RescheduledException: Build of instance 85db4722-bdd9-4004-bf7a-ff8d0c5582d6 was re-scheduled: No disk at /var/lib/nova/instances/_base/ephemeral_1_0706d66\n']