LU-1804 tests: skip OI scrub speed test under UP env

LU-1804 tests: skip OI scrub speed test under UP env

The OI scrub speed depends on CPU/RAM schedule. There may be more
schedule delay under UP environment. So skip related test cases.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: Iebc514857c6d88601598a7da58a340296655dcf1
Reviewed-on: http://review.whamcloud.com/3846
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>