LU-11467 utils: add lfs mirror delete command

LU-11467 utils: add lfs mirror delete command

Add "lfs mirror delete" as an alias for "lfs mirror split -d", to
balance "lfs mirror create" and simplify the interface for users.

Add lfs-mirror-create.1 man page, and convert some tests in
sanity-flr.sh to use the new interface.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Change-Id: I4399878dc2fd435c517a2ff529b91480583ebbe5
Reviewed-on: https://review.whamcloud.com/36185
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>