Replace fixtures.DetailStream with fixtures.StringStream.
Replace fixtures.DetailStream with fixtures.StringStream.

Fixtures 0.3.12 adds StringStream as a successor to DetailStream (this
change was made for compatibility with python 3). Update Nova's tests to
use StringStream instead of DetailStream.

Change-Id: If697859a52f3a7db9dab38bd080b779bb3270287

hyakuhei/cleantox,openstack-dev/hacking,emonty/oslo-hacking,zancas/hacking,hyakuhei/cleantox,openstack-dev/hacking,zancas/hacking,emonty/oslo-hacking