LU-1897 test: replay-single test_70b dbench not found

dbench is taking too long to start. It was observed
that a 12 seconds sleep was not long enough to start
dbench on all clients.

I added a loop to rally all clients before starting
the testing and improved the error messages.

Signed-off-by: Keith Mannthey <keith.mannthey@intel.com>
Change-Id: I7b0fce4c39c003c4db23686b295fb5e45a6f8a92
Reviewed-on: http://review.whamcloud.com/5761
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>