LU-13156 tests: skip sanity-selinux test_21a+21b

LU-13156 tests: skip sanity-selinux test_21a+21b

These tests are failing most of the time, but are the last failures
for both review-dne-selinux and review-dne-selinux-ssk sessions of
this script.  Add them to ALWAYS_EXCEPT so that these sessions can be
enforced to prevent further regressions.  They can be fixed afterward.

Test-Parameters: trivial testlist=sanity-selinux
Test-Parameters: testlist=sanity-selinux env=SHARED_KEY=true
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ib51e3749b1e262336ae1db53278909dd78d2be63
Reviewed-on: https://review.whamcloud.com/37899
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>