tests: Wait for nfs export to be available

Change-Id: I59a5e0cb78f2b670761a65272b8ab1d7bdb3668a
BUG: 1092850
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/7773
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>