Make test_different_fname_concurrency less racy

This test case makes a bad assumption about when a greenthread will run
and signal its done event.  The test code yields once using a sleep and
expects that the done event will get signaled before the test code runs
again.  This may not be the case.  Instead, yield up to 10 times waiting
for the done event to occur.

I was not able to reproduce this failure locally, but we're seeing it
regularly in CI.  It seems that this should resolve it.

Change-Id: I978ae9eef5bb1ac296055dc1613406e4284a625f
Closes-bug: #1270654
(cherry picked from commit a35776d337493b14d2937f0f4a03aa4b119d018b)
Merge "Make test_different_fname_concurrency less racy" into stable/grizzly
Merge branch 'stable/grizzly' of https://github.com/openstack/nova into grizzly/stable