glusterd: Help does not show performance.nfs.* opt

Gluster does not display performance.nfs.* options in help.

In Gluster NFS, write-behind is the only performance xlator
which gets loaded. Gluster volume set help should display all
the options provided by write-behind xlator.

Change-Id: I4a41151a6c15eeed8e8d123a6044c6f0c42b56b0
BUG: 1090826
Signed-off-by: Santosh Kumar Pradhan <spradhan@redhat.com>
Reviewed-on: http://review.gluster.org/7546
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>