Sleep to give logs a chance to be flushed.

The Jenkins SCP plugin copies log files that have been copied by
devstack-gate from their normal locations to the log directory for that
particular test run. Sometimes these SCP'ed files are showing up
incomplete and that appears to be happening because the services are
unable to flush their log buffers before copying happens. Sleep for two
seconds before copying the logs to give the log buffers a chance at
flushing.

Change-Id: I12307a2f93839015e83389da8f6a10549044bd81