task/nfs: stop nfs server after test finishes more thoroughly

In addition to "exportfs -au", we take a sledgehammer to it
by shutting down all the worker threads and flushing out
the client export table.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Greg Farnum <Greg@inktank.com>
Merge pull request #342 from ceph/wip-5876

task/nfs: stop nfs server after test finishes

Reviewed-by: John Spray <john.spray@redhat.com>