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
Merge "Fix cisco n1kv plugin cli option and help"