LU-9968 tests: correct stripe index sanity 300g

LU-9968 tests: correct stripe index sanity 300g

In sanity test 300g, we set the starting stripe index to MDT 1
and MDT 2 using 'lfs setstripedir -iN' for N = 1 and 2. At the
beginning of the test, we check that there are two or more MDTs
in the Lustre file system being tested. If there are only two MDTs,
then this test will fail when we try to set the starting stripe
index to 2 because the MDT indexes are zero based.

For santiy test 300g only use MDT start index 0 and 1.

Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: If2c252ad9bb7249aa777764f212ee40523aee82f
Reviewed-on: https://review.whamcloud.com/28935
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Saurabh Tandan <saurabh.tandan@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>