Hook scripts should exit with correct exit status.

When volume set options other than nfs-ganesha.host and nfs-ganesha.enable
are executed, the hook script S31ganesha-set.sh should exit with
exit status 0. The present behaviour results in an error message in
one of the logs that could be  misleading.

Similarly, hook script S30samba-set.sh should exit with
exit status 0 if samba is not installed. Fixing the issue
with a minor change in the script.

Change-Id: I35d1dc8400bb7043385800ed97adc5321829437f
BUG: 1122978
Signed-off-by: Meghana Madhusudhan <mmadhusu@redhat.com>
Reviewed-on: http://review.gluster.org/8385
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>