SDK Refactor: Prepare network segment commands
SDK Refactor: Prepare network segment commands

Prepare the OSC "network segment" commands for the SDK refactor.
See [1] for details.

[1] https://etherpad.openstack.org/p/osc-network-command-sdk-support

Change-Id: I426ecdad0f9d95b89e80fb1c5ba81ffe5fe274c9
Partially-Implements: blueprint network-command-sdk-support

dtroyer/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient