LU-3285 lov: add MDT target to the LOV device

LU-3285 lov: add MDT target to the LOV device

MDC becomes LOV target like OSC for Data-on-MDT needs.
Patch does the following:
- new composite layout entry type is added - LLT_DOM to
describe Data-on-MDT striping.
- LOV process config log and checks for MDC targets organizing
them separately from OSCs
- LOV operations are changed where needed to understand new layout
entry type

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I8a985d66a5f283ed387a311ff46f307c60317a79
Reviewed-on: https://review.whamcloud.com/28010
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>