OS-438, [v3] IPv6 support for Infoblox

Code was rebased
>BTW, it just occur to me - before we spawn a new process, should we kill existing process (with the exact same cmd) just in case they were stale?

Currently, during dhcp_agent sync_state we are killing exists processes by pid. But it's possible to expand dhcp_relay:disable to also check if same command exist in process list. But lets do it as separate commit.