A test for concurrency when glance uses sleep.

Bug 988099 showed that time.sleep was not being monkey patched
resulting in no concurrency when a sleep was called.  This was
fixed with change I24dfe818.  This commit provides a regression
test to ensure that this behavior remains in place as glance is
developed over time.

Change-Id: Iafbfaa9fd672b5ffd1d19a43dee03dfbfc6d3a2d
Merge "A test for concurrency when glance uses sleep."