OS-438, [v3] IPv6 support for Infoblox

Code was rebased
>Is there absolutely no way to get the pid from the netns.execute() and utils.execute() above? It seems odd that we have to figure out what the pid is inside _save_process_pids() below by scanning /proc
>Not only that it is inefficient, it would be possible to, for example, pick up the same process that was started previously.

Good idea btw, i just checked dhcrelay man page, and they also have custom pid file. Do not know why guys who wrote it didn't used it

       -pf pid-file
              Path to alternate pid file.

I will rewrite whole this part to make it simple.