LU-17431 tests: exercise dynamic nodemap on MDS

LU-17431 tests: exercise dynamic nodemap on MDS

Rename sanity-sec test_72 to test_72a to exercise dynamic nodemaps
on OSS side. And add sanity-sec test_72b to exercise dynamic nodemaps
on MDS side. This supports the case where the MDS runs on the same
node as the MGS, and with a combo MGS/MDS as well.

Test-Parameters: trivial
Test-Parameters: testlist=sanity-sec env=ONLY=72 mdscount=2 mdtcount=4 osscount=1 ostcount=8 clientcount=2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Idb32ee46f8809160ddc4c54960c10d565b3396d7
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55002
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>