LU-1018 tests: reduce runtime value for compilebench

We are reducing the cbench_IDIRS and cbench_RUNS to 2.
These variables can be adjusted to other runs beside
sanity check.

Test-Parameters: envdefinitions=SLOW=yes,ENABLE_QUOTA=yes testlist=parallel-scale
Signed-off-by: Minh Diep <minh.diep@intel.com>
Change-Id: If03f7b5a71eb8a4b4b380638af872a012f9cdac5
Reviewed-on: http://review.whamcloud.com/5052
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>