LU-14928 mgs: allow md target re-register

LU-14928 mgs: allow md target re-register

In a DNE system, it is not safe to do writeconf of
a MD target and attempt to mount (and re-register) it again,
as it creates a weird MDT-MDT osp devices like
fsname-MDT0001-osp-MDT0001" and makes the system non-functioning.
The fix doesn't allow creation of illegal devices.

HPE-bug-id: LUS-10098
Signed-off-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: I698ee6d70ac96f54eaec57b5c5fe553d130ba011
Reviewed-on: https://review.whamcloud.com/44594
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Artem Blagodarenko <artem.blagodarenko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>