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
Update cliff version to 1.4.3 in requirements.txt

cliff 1.4.3 fixed the encoding output issues,
So update cliff version in file requirements.txt

Fixes bug #1189112

Change-Id: If3bd7da80b204c41c714fd68373be6798b322e8d
Merge "Fix cisco n1kv plugin cli option and help"
Merge "Update cliff version to 1.4.3 in requirements.txt"