[agent] Post prepare network changes should restart agent [fixes #52275897]

When there's a network change on an existing vm, director sends a prepare_network_change message to the vm
agent. After agent replies to director with a `true` message, the post_prepare_network_change method is called
(via EM callback).

The post_prepare_network_change  method will delete the udev network persistent rules, delete the agent settings
and then it should restart the agent to get the new agent settings (set by director-cpi). For a simple network
change (i.e. dns changes) this is enough, as when the agent is restarted it will apply the new network settings.
But for other network changes (i.e. IP change), the CPI will be responsible to reboot or recreate the vm if needed.