extras/hook-scripts: Don't add multiple entries in smb.conf

When volume is force started, there may be entry in smb.conf
for that volume. Always check for entry before adding one.

Change-Id: I44f893174a993abaa31c24c1a81bb38a58617808
BUG: 994727
Signed-off-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-on: http://review.gluster.org/5526
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>