gluster vol set nfs-ganesha.enable off fails to unexport volume.

 Due to a parameter type change in upstream nfs-ganesha
( v2.1 and above ), gluster vol set <volname> nfs-ganesha.enable
 off doesn't unexport the volume in question if it was already
exported.
A minor change in the hook script would result in the
expected behaviour.

Change-Id: I46f76f3cde6b6ebd7a94d5ee16f05a3648a2840f
BUG: 1124711
Signed-off-by: Meghana Madhusudhan <mmadhusu@redhat.com>
Reviewed-on: http://review.gluster.org/8390
Reviewed-by: soumya k <skoduri@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>