prune: Use summary.yaml to determine age of job
prune: Use summary.yaml to determine age of job

If a job has been partially pruned, its directory's mtime will get
bumped, causing total removal to be delayed by potentially months.
Looking at the mtime of summary.yaml should provide results that are
more intuitive.

Signed-off-by: Zack Cerza <d7cdf09fc0f0426e98c9978ee42da5d61fa54986@redhat.com>

dmick/teuthology,SUSE/teuthology,ktdreyer/teuthology,SUSE/teuthology,SUSE/teuthology,ceph/teuthology,ceph/teuthology,ktdreyer/teuthology,dmick/teuthology,dmick/teuthology