tests: Increase wait time for nfs mount.

Change-Id: I61815b502c90314ea6924e3046fb9b396ff56e8b
BUG: 927616
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/5051
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>