LU-7103 test: avoid cat of /dev/urandom

LU-7103 test: avoid cat of /dev/urandom

Use a different command to generate a random number.
Using cat /dev/urandom doesn't work in all cases.

Test-Parameters: clientdistro=el7
testlist=ost-pools,ost-pools,ost-pools,ost-pools,ost-pools,ost-pools,ost-pools,
ost-pools,ost-pools,ost-pools,ost-pools,ost-pools,ost-pools,ost-pools

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I63cfad2654cf654460263529457538965e373f82
Reviewed-on: http://review.whamcloud.com/17350
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>