From RFC 3530:
  During the grace period, the server must reject READ and WRITE operations
  and non-reclaim locking requests (i.e., other LOCK and OPEN operations)
  with an error of NFS4ERR_GRACE.

That basically means that all open() calls from clients are blocked until the
grace period is over (90 seconds by default).

We want the grace period as low as possible to speed up the tests. '10'
seconds were tested on Fedora 21 and Ubuntu 15.04 as the hosts.

The test image is rebased to Fedora in order to get 'rpc.nfsd -G <n>' option,
Ubuntu does not support it.