OS-438, [v3] IPv6 support for Infoblox

Code was rebased
I would suggest here not just to check if pid directory exists, but also check if this pid belongs to dhcrelay by reading /proc/%s/cmdline.
'cmdline' contains command line by which process was started, for our case will look like:
/usr/local/dhcp-4.3.0/sbin/dhcrelay-a-itapd379bce-ltrel556e1f192.168.124.159
So you could verify if it starts with  'self.conf.dhcrelay_path'

Otherwise we can end up with killing processes which does not belong to us by sending them SIGKILL.