LU-911 lut: few more methods from orion are taken

LU-911 lut: few more methods from orion are taken

target.c is updated from orion:
- has own environment key now
- int lut_client_del
- int lut_client_add
- int lut_client_new

MDT related code is cleaned up

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I07a5f1f11286004ec957587fdf16fcd5a4dd073f
Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1849
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>