Fix cisco n1kv plugin cli option and help

Rename VXLAN option to OVERLAY and add help for sub-type field in
the Cisco N1kv specific cli to accomodate
cisco-plugin-n1k-enh-vxlan-support blueprint.

Change-Id: I4a3dc7ad1b45f6effebd6fd337c51e7c0eeba84c
Closes-Bug: #1220966
This is a duplicate commit in 2.3.0-n1k-havana-dev of
7932447a633247408530c7baa99b055f52f7e882 (which originated in 2.3.3-n1k-havana-dev)

This duplicate commit is for Cananicol lineup purpose only

for more info https://bugs.launchpad.net/springfield/+bug/1278673