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.

Whamcloud-bug-id: ATM-1283
Test-Parameters: clientselinux testlist=sanity,recovery-small,sanity-selinux
Test-Parameters: envdefinitions=SHARED_KEY=true testlist=sanity,recovery-small,sanity-sec
Test-Parameters: envdefinitions=SHARED_KEY=true clientselinux testlist=sanity,recovery-small,sanity-selinux,sanity-sec
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1016a459c42ffed1ab2b6f67d0a145ed2af9fa40
Reviewed-on: https://review.whamcloud.com/34521
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>