Use _get_columns() to obtain columns in network.py
Use _get_columns() to obtain columns in network.py

Objects returned by Network v2 and Compute v2 are different.
When getting columns to display, Network v2 uses obj.keys(),
while Compute v2 uses obj._info.keys(). But both of them could
obtain the keys of the object by _get_columns().

Change-Id: I347815f2d28822a95bd6f57d429b84b7ca96e0ee

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