LU-16708 tests: fix zconf_mount_clients for SSK

LU-16708 tests: fix zconf_mount_clients for SSK

When SHARED_KEY is in use, zconf_mount_clients can add 'skpath' mount
option to load nodemap-specific keys. But this must not overwrite
already specified mount options.

Fixes: 05e5cb0b0c ("LU-16683 tests: fix sanity-sec test_61 for SSK")
Test-Parameters: trivial
Test-Parameters: testlist=sanity-sec env=ONLY=61
Test-Parameters: testlist=sanity-sec env=SHARED_KEY=true,ONLY=61
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I3dbc6ffe72722659ead77fb1c2c2675873c7aff2