LU-202 fix the test_23 of sanity-quota.sh

LU-202 fix the test_23 of sanity-quota.sh

test_23 uses a constant value(120000 = ~120M) to check if the system has
enough space to finish the test, so in the large OSTCOUNT testing
environment, the test will fail for short of disk space.

Changes of this patch are:
- Use a reasonable value to check free space instead of magic 120000;
- Specify the stripe index of test file for test_9/10 and test_23;
- Check free space over all OSTs, and select the ost which has enough
  space to do test. (test_9/10/23)

Issue:
Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I2d9059d23cf53fee0244a226622b0e9adb800599
Reviewed-on: http://review.whamcloud.com/454
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann@whamcloud.com>