LU-13970 tests: skip sanity test_427 on SLES12

LU-13970 tests: skip sanity test_427 on SLES12

For SLES12 sanity.sh test_427 cannot determine the number of
objects in the lustre_inode_cache slab, so skip this old distro.

Test-Parameters: trivial testlist=sanity env=ONLY=427 clientdistro=sles12sp5
Fixes: acbc5b5bb2b5 ("LU-13970 llite: add option to disable Lustre inode cache")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ia6f2d57969d83d763ed91251dfaaf9929c3ebbe5
Reviewed-on: https://review.whamcloud.com/41851
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>