LU-16775 tests: cleanup target after sanity-sec test_31

LU-16775 tests: cleanup target after sanity-sec test_31

sanity-sec test_31 adds an LNet network tcp999, and associated
servicenode param on MDS target. This param must be cleared when
exiting the test, otherwise it can lead to incorrect client HA
behavior, trying to reach out to the fake service nodes.

Test-Parameters: trivial
Test-Parameters: mdscount=2 mdtcount=4 osscount=1 ostcount=8 clientcount=2 clientselinux testlist=sanity-sec env=SHARED_KEY=true,ONLY="31 61"
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: If3a1926855bd23e9154c9a32b7a555e934e94565