LU-160 check the minimum available size of OST in test_155_load()

LU-160 check the minimum available size of OST in test_155_load()

Fix test_155_load() in sanity.sh to check whether the minimum available
size of OST is bigger than the large file size. If not, then sanity test
155{a,b,c,d} would be skipped.

Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I0a587337ad188a96f2ada51726604029c666df07
Reviewed-on: http://review.whamcloud.com/368
Tested-by: Hudson
Reviewed-by: Prakash Surya <surya1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>