task/nfs: stop nfs server after test finishes more thoroughly
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 <5a3288024fcad83740d827e0862f4c1e6f28e7bf@redhat.com>
Signed-off-by: Greg Farnum <bc65ce8cc316901614469d203ed8eb95ff62d502@inktank.com>

dreamhost/teuthology,dmick/teuthology,ceph/teuthology,michaelsevilla/teuthology,dreamhost/teuthology,ktdreyer/teuthology,tchaikov/teuthology,michaelsevilla/teuthology,tchaikov/teuthology,caibo2014/teuthology,t-miyamae/teuthology,yghannam/teuthology,zhouyuan/teuthology,zhouyuan/teuthology,dmick/teuthology,SUSE/teuthology,ceph/teuthology,yghannam/teuthology,ktdreyer/teuthology,robbat2/teuthology,robbat2/teuthology,SUSE/teuthology,ivotron/teuthology,dmick/teuthology,SUSE/teuthology,t-miyamae/teuthology,caibo2014/teuthology,ivotron/teuthology