EX-2797 lpurge: initial support for DoM

EX-2797 lpurge: initial support for DoM

lpurge should be able to scan MDT device, recognize objects with DoM
component and remove a replica with DoM component if another in-sync
replica exists.

Test-Parameters: testlist=hot-pools
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: If12e0448ab07527d86832d942a63b4a0189ad7a0
Reviewed-on: https://review.whamcloud.com/43405
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Tested-by: John L. Hammond <jhammond@whamcloud.com>