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*.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ic2bedb4cc12e9adad0ce63bd0617b2e0ec13907e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50377
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Jonathan Calmels <jcalmels@nvidia.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>