ceph: debug valgrind error

  File "/var/lib/teuthworker/teuthology-master/teuthology/task/ceph.py", line 215, in valgrind_post
    (file, kind) = line.split(':')
ValueError: need more than 1 value to unpack
ceph: fix valgrind grep output parsing

When you pass a single file to zgrep you don't get the filename prefix,
which confuses the split line a few lines down.