LU-9341 tests: skip interop sanity test_27U

LU-9341 tests: skip interop sanity test_27U

Skip sanity test_27U for MDS less than 2.15.51
append pool was added by LU-9341 and test_27U was
added by LU-15727

Fixes: e2ac6e1eaa ("LU-9341 lod: Add special O_APPEND striping")
Fixes: 0396310692 ("LU-15727 lod: honor append_pool with default composite layouts")
Test-Parameters: trivial testlist=sanity env=ONLY=27U serverversion=2.14.0

Change-Id: I5c9d2ebf4e1e660fa60d845c9894409f0e96e01f
Signed-off-by: Wei Liu <sarah@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49935
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>