task/ceph_fuse: hopefully cleaner umount

Something I don't quite understand went wrong
with the umount stage in ceph_fuse.  It was timing out
in run.wait(), even though the log showed the
"fuse finished with error 0" message.  I'm suspicious
that there is something subtle going on here with
checking finished on RemoteProcess rather than
calling wait(), as I've seen cases where finished
doesn't accurately pick up termination.

Signed-off-by: John Spray <john.spray@redhat.com>