cli/hooks : Add volume set options to enable/disable nfs-ganesha support.

1. gluster volume set nfs-ganesha.enable ON/OFF
If the option is set to ON, the volume field in the nfs-ganesha configuartion file is
edited. Gluster-nfs is disabled on that volume and the volume is exported using
nfs-ganesha.

2.gluster volume set nfs-ganesha.host IP
This is used to provide the IP of the nfs-ganesha host.

Note : nfs-ganesha.host MUST be set before using nfs-ganesha.enable ON

The switch from gluster-nfs to nfs-ganesha is mostly done by the hook-scripts
in the post phase of the 'set' option. As a result, gluster volume reset does not
function as it is expected to. By default, nfs-ganesha will be set to off but the
process  will not be killed.

Hence, a few changes have to be made post 'reset' option as well. Those changes
also have been added.

Change-Id: I7fdc14ee49d1724af96eda33c6a3ec08b1020788
BUG: 1092283
Signed-off-by: Meghana <mmadhusu@redhat.com>
Reviewed-on: http://review.gluster.org/7321
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>