tests: Ensure portmap registration before nfs mount.

Change-Id: I12a660a7dfbe4a2d0428910d762434043395fe02
BUG: 927616
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/5012
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>