add static routes definition

Right now we have some sad static routes, that we have to add manually after reboot. We should add option to add persistent static routes.

Example:

networking_routes:
  - network: "192.168.0.0/24"
    via: "10.0.0.7"
    device: "eth7"

This configuration will add one static route similar to command: ip route add 192.168.0.0/24 via 10.0.0.7 dev eth7

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