LU-13156 tests: wait for nodemap update in sanity-selinux

LU-13156 tests: wait for nodemap update in sanity-selinux

In sanity-selinux test_21a and test_21b, nodemaps are used to test
SELinux status checking (sepol).
We must wait for nodemap update on all MDS nodes before carrying out
tests.

Lustre-change: https://review.whamcloud.com/38034
Lustre-commit: f1761cbe6b1243edd7a69c68c401d7285f7f3b38

Test-Parameters: clientselinux mdscount=2 mdtcount=4 testlist=recovery-small,sanity-selinux env=ONLY="21 23",ONLY_REPEAT=80
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I363e2bec757efc199f7039f8af4bcb77e2a2a184
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50400
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>