LU-6710 tests: Constrain stripe index conf-sanity test 82a

LU-6710 tests: Constrain stripe index conf-sanity test 82a

conf-sanity test 82a specifies the OSTs to stripe a file over.
The OST index is computed as RANDOM * 2 for a maximum of 65534.
Yet, the maximum stripe count is 65532. Thus, the OST index in
conf-sanity test 82a needs to be limited.

Also, change the single use of the deprecated
llapi_stripe_offset_is_valid() to llapi_stripe_index_is_valid().

Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: I4ceb44d639b88527105c1e8812cbd7590d041316
Reviewed-on: http://review.whamcloud.com/15824
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Saurabh Tandan <saurabh.tandan@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>