LU-12131 tests: fix SSK handling in tests

LU-12131 tests: fix SSK handling in tests

SSK can be activated for Lustre tests by setting SHARED_KEY env
variable to true.
In setup_all() an additional env variable SK_MOUNTED is used to avoid
mounting an SSK file system twice. But this variable has to be set
back to false in stopall() for consistency.
Some tests are incompatible with SSK, so skip them in case SHARED_KEY
is true. Some other tests playing with nodemaps have to take SSK into
account.

Lustre-change: https://review.whamcloud.com/34521
Lustre-commit: ee6904e312f7d7446f390cff0ec3c6e48b98e32b

Whamcloud-bug-id: ATM-1283
Test-Parameters: envdefinitions=SHARED_KEY=true testlist=sanity,recovery-small,sanity-sec
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1016a459c42ffed1ab2b6f67d0a145ed2af9fa40
Reviewed-on: https://review.whamcloud.com/35556
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>