LU-16646 krb: use system ccache for Lustre services

LU-16646 krb: use system ccache for Lustre services

Instead of hard-coding a FILE credentials cache for Lustre services,
comply with the system configuration in place and use the default
ccache.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ib89fe117e9f1d937925a02c7ed786a81cd8954cb
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50342
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>