gNFS : mount fails if connection to brick(s) is not established yet.

       Connection between gluster-nfs and brick process requires
       time (especially for rdma).During that time when we try to
       mount using nfs ,it will fail saying that volume not found.
       So we need a check using 'is_nfs_export_available'before
       mounting to ensure volume is available.This patch will provide
       the check before nfs mount if it is not given in the test files

Change-Id: I242eb6e3118ebaca1df46314302a203a0c9738a8
BUG: 1158831
Signed-off-by: jiffin tony thottan <jthottan@redhat.com>
Reviewed-on: http://review.gluster.org/9011
Reviewed-by: Meghana M <mmadhusu@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>