Restarting agent service triggers re-registration (sanity checks)

This was reported by @marcin during 0.15 sanity checks:

When the user restarts the agent service:

$ systemctl service restart isc-agent-service

the registration starts over. The agent which used to be authorized is now back to unauthorized agents and again requires an approval. This is not right. We should have a way to determine that the agent has been already registered and do not re-register. Maybe simply trying to ping itself via the server would be sufficient to see that the connection can be established.