Make use of oslo.privsep to support namespaces. This includes all
relevant code necessary for oslo.privsep to work.
Change ip_lib's get_routing_table method to use pyroute2, rather than
parsing the output of 'ip route'.
Change-Id: I89bfa3dbf1776da973cfca389b2841019a520f75
Partial-Bug: 1492714
Co-Authored-By: Angus Lees <gus@inodes.org>