LU-3326 tests: limit the disk space used by run_*.sh

In run_dd.sh and run_tar.sh, limit the disk space to be used
according to the total free disk space and the client counts.

Test-Parameters: envdefinitions=ENABLE_QUOTA=yes \
clientdistro=el6 serverdistro=el6 clientarch=x86_64 \
serverarch=x86_64 \
clientcount=4 osscount=2 mdscount=2 \
austeroptions=-R failover=true useiscsi=true \
testlist=recovery-mds-scale

Change-Id: I61ea1167906312fdb182b67d7169e95cad1b6d72
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: http://review.whamcloud.com/6376
Tested-by: Hudson
Reviewed-by: Minh Diep <minh.diep@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>