LU-80 utils: add "-c <stripe_count>" option to lfs_migrate

LU-80 utils: add "-c <stripe_count>" option to lfs_migrate

Add "-c <stripe_count>" option to lfs_migrate to support
restriping the file with a specified stripe count. A sanity
test case was also added to check this option.

Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: Ia86d2417cfa7d08a424ef325c9b3a4509abfc87f
Reviewed-on: http://review.whamcloud.com/2247
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>