LU-16799 tests: fix sanity-krb5

LU-16799 tests: fix sanity-krb5

sanity-krb5.sh needs to be fixed in several ways.
It cannot assume that the Kerberos credentials cache is FILE, and
expect ccache files to be under /tmp/krb5cc_*.
The lsvcgssd daemon must be launched with -vvv flags for easier
debugging.
Keyring needs to be cleared appropriately after using 'lfs flushctx'.

Test-Parameters: trivial testlist=sanity-krb5 kerberos=true
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I31ca8d2d97e137c7ba9fa478d5432aeedb5135a8
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50864
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>