LU-17173 tests: fix security related tests

LU-17173 tests: fix security related tests

Several cleanups required in security related tests.

In sanity-krb5, in order to get proper access to keyrings, use su -
instead of runas to initialize process more completely.
Also fix use of 'lfs flushctx', as some tests do not call it properly.
And in test_8, avoid waiting arbitrarily and change fail_loc to just
sleep once.

In sanity-krb5 and sanity-sec, fix parameters passed to
start_gss_daemons().

Lustre-change: https://review.whamcloud.com/53012
Lustre-commit: 9fc12ca7f29bd70be19471c2b9143d50d2e24eda

Test-Parameters: trivial
Test-Parameters: kerberos=true testlist=sanity-krb5
Test-Parameters: testgroup=review-dne-selinux-ssk-part-2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I4598ae5a7d28afbc39d7cc2d0afd1096d877d03b
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53910
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>