Ubuntu: support policy-based routing in systemd-networkd

This change adds support for policy-based routing via systemd-networkd.
Due to differences in the configuration mechanism, routing policy
rules are configured via dicts for Ubuntu, while remaining as strings
on CentOS. Ideally we would support both formats.

Story: 2004960
Task: 42217

Change-Id: I77aec0160eb7e4dd763326bfe6e3d9a44b248108