LU-12131 tests: only create lgssc.conf file if necessary

LU-12131 tests: only create lgssc.conf file if necessary

lgssc.conf file is now packaged by Lustre, and installed under
/etc/request-key.d/.
So, unless run from build tree, init_gss() must not create its own
anymore. So adjust corresponding commands in init_gss() and
cleanup_sk().

Lustre-change: https://review.whamcloud.com/34520
Lustre-commit: 66919f2b687f8b15679e6ff4e22a3f66f7d1c13a

Fixes: e299df1e9eea ("LU-7854 gss: install lgssc.conf under /etc/request-key.d")
Whamcloud-bug-id: ATM-1283
Test-Parameters: envdefinitions=SHARED_KEY=true testlist=sanity-sec
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I9cc76fddb8a622d7c40d6348913df42ae063254a
Reviewed-on: https://review.whamcloud.com/35557
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>