LU-160 Reduce OST size requirement for test 155

LU-160 Reduce OST size requirement for test 155

The OST size requirements for sanity test 155 is reduced by running with
only a single-striped file on a particular OST, and only checking the
RAM on that OST's OSS node. Because it is no longer trying to create a
file larger than multiple OSS nodes' RAM while only striped over a
single OST, these tests should run at times when they previously would
not have.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: I4c1829240f4cd44366ad0de24ef6caa444f98801
Reviewed-on: http://review.whamcloud.com/534
Tested-by: Hudson
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Chris Gearing <chris@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>