LU-17996 mgs: add ability to clear exports

LU-17996 mgs: add ability to clear exports

Just like with other targets (MDT, OST), give the ability to clear
dead exports from the exports list 'mgs.MGS.exports'.
Improve sanity-sec test_31 to benefit from this new ability.

Test-Parameters: trivial
Test-Parameters: testgroup=review-dne-part-2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I4e99de31834753d223fd3cfe226f6f0343f2586b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55596
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>