LU-1458 tests: wait for dbench to start

Use wait_for_function instead of just guessing that 20s will be long
enough for dbench to start.  Wait for up to 6min in 5 second
intervals.  This also ensures that dbench will run for a little while
after it starts.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ic9c61b487c4013cccd620491d617c10e2206fd62
Reviewed-on: http://review.whamcloud.com/7914
Tested-by: Hudson
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>