Better healthchecks for system tests
Our system tests are a bit more unstable than some time ago. In my opinion, it is related to downgrading SupervisorD last time.
It seems that the older version doesn't wait for Kea is not operational.
We should analyze the possibility of using custom checks to test if Kea is running and has all necessary sockets opened.
It'll improve the stability of tests that depends on running the Kea DHCP daemon.