LU-17050 tests: test Kerberos env in sanity-krb5

LU-17050 tests: test Kerberos env in sanity-krb5

Test Kerberos environnement is sane before trying to launch
sanity-krb5 tests.

Lustre-change: https://review.whamcloud.com/52068
Lustre-commit: TBD (from 8e8dc1b7e715f46b234ae0b018e2ccb464658df4)

Test-Parameters: trivial kerberos=true testlist=sanity-krb5
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1675ba7db8c62687c69359a15cc931b5dfd40018
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52150
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>