New port: net-mgmt/py-nxapi-plumbing: Low-level library for managing Cisco

devices through NX-API

Low-level library for managing Cisco devices through NX-API using JSON-RPC
and XML.

It's the successor of net-mgmt/py-pynxos with a more open license. Thus this
port has been repocopied from net-mgmt/py-pynxos .

WWW: https://github.com/ktbyers/nxapi-plumbing

Approved by: miwi (mentor)
Differential Revision: https://reviews.freebsd.org/D19288