LU-15282 tests: improve sanity test_51d coverage

LU-15282 tests: improve sanity test_51d coverage

Improve sanity test_51d to test all different stripe counts, rather
than only striping over all OSTs.  With the current default test
config there are 7 OSTs, and this does not cover some test cases.

Test-Parameters: trivial testlist=sanity env=ONLY=51d ostcount=3
Test-Parameters: testlist=sanity env=ONLY=51d ostcount=4
Test-Parameters: testlist=sanity env=ONLY=51d ostcount=5
Test-Parameters: testlist=sanity env=ONLY=51d ostcount=6
Test-Parameters: testlist=sanity env=ONLY=51d ostcount=7
Test-Parameters: testlist=sanity env=ONLY=51d ostcount=8
Test-Parameters: testlist=sanity env=ONLY=51d ostcount=9
Test-Parameters: testlist=sanity env=ONLY=51d ostcount=10
Test-Parameters: testlist=sanity env=ONLY=51d ostcount=11
Test-Parameters: testlist=sanity env=ONLY=51d ostcount=12
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Icc694d037e53f7bf966aff8ca1070d42ac3ebbe5
Reviewed-on: https://review.whamcloud.com/46154
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>