LU-1303 lod: methods to manage pools

LU-1303 lod: methods to manage pools

external components (like configuration) call these
methods to initialize and manage in-core list of
pools in lod. so that lod itself doesn't need to
implement store/load functionality.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I6b8bbc55b9acf116cc8c06e756ad26b327b5d4e8
Reviewed-on: http://review.whamcloud.com/4047
Reviewed-by: wangdi <di.wang@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>