LU-16646 krb: improve lookup of user's credentials

LU-16646 krb: improve lookup of user's credentials

Rather than only looking up for user's credentials in hard-coded
FILE:/tmp/krb5cc_<uid>, try first the default ccache on the system,
and then fallback to files matching /tmp/*krb5cc* and
/run/user/<uid>/*krb5cc*.

Lustre-change: https://review.whamcloud.com/50377
Lustre-commit: 4b12f2b9ae14556285529b40985a7c3aacdbb7f4

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ic2bedb4cc12e9adad0ce63bd0617b2e0ec13907e
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50653
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>