LU-12472 tests: update sanity-krb5.sh

With current Lustre version, some behaviors have changed.

When a user tries to access Lustre without valid Kerberos credentials,
access is denied, and the key generated in the user's session keyring
is marked as revoked. This key must be unlinked so that further
access with valid Kerberos credentials regenerates a new, valid key in
the user's session keyring.

When Kerberos credentials presented by the user cannot be validated
on server side because the lsvcgssd daemon is down, client's request
immediately fails instead of being blocked until lsvcgssd daemon
is up and runnning again.

*gssd daemon is automatically started in the background, no need for a
'&' on the command line.

Test-Parameters: trivial
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1b929ac593fd427c199cffa02d364b40426d32bf
Reviewed-on: https://review.whamcloud.com/35317
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>