LU-12131 tests: fix test_802 for GSS

LU-12131 tests: fix test_802 for GSS

test_802 should not overwrite already existing client mount options
when trying to mount client as read-only.

Lustre-change: https://review.whamcloud.com/35335
Lustre-commit: a51d0653cf46fc898da01f86c26cc0f4f5beff5a

Test-Parameters: trivial
Test-Parameters: envdefinitions=ONLY=802 testlist=sanity
Test-Parameters: envdefinitions=SHARED_KEY=true,ONLY=802 testlist=sanity
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I8189c245870fb0caf48006db11621f0af48e1878
Reviewed-on: https://review.whamcloud.com/35535
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>