LU-12785 dom: fix DoM component deletion code

The lod_erase_dom_stripe() deletes DoM entry from composite
layout upon file create if DoM is disabled on server.
That code works incorrectly if DoM is not the first component
in provided layout, e.g. in mirror.

Patch does correct DoM entry removal in generic case no matter
where it was placed in layout. Related test 270h is added into
sanity.sh

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ia1b3f25db16a7b59b83cd8f58ff44ddf082cab48
Reviewed-on: https://review.whamcloud.com/38337
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>