LU-1670 tests: Load in-tree osd-zfs when necessary

LU-1670 tests: Load in-tree osd-zfs when necessary

In a developer's environment, Test Framework should try loading osd-zfs
modules from Lustre build trees when ZFS-based targets are used.

Change-Id: I47646738810401c0a48ecc5ca1459952ff811dce
Signed-off-by: Li Wei <liwei@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/3458
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>