Fix broken pipe error on xenial integration tests
It appears that the manager.close_all() can cause a broken pipe
error.  At this stage, that means that the pipe is closed and thus
it's actually okay to just continue and ignore the error (as per stdin)
Fixes:
@@ -438,7 +438,10 @@ class Instance(model.Model):
stdout.finish_soon()
stderr.finish_soon()
+ try:
manager.close_all()
+ except BrokenPipeError:
+ pass
while not stdout.finished or not stderr.finished:
time.sleep(.1) # progma: no cover
