OS-438, [v3] IPv6 support for Infoblox

Code was rebased
This version looks workable, however I would update this part to be more cpu cycle effective.
It is not performance critical place, but it worth to do it better because we usually have more than 200 items in /proc/.

Currently we are scanning whole cmd line for 'interface_name' twice, it is better to avoid doing so.
startswith() check is much more cheap than searching one line in another, so this check should go first.
So as I see code should look like:
# do it before 'for' block since we need to calculate it only once
path_length =  len(self.conf.dhcrelay_path)
# 
if cmdline.startswith(self.conf.dhcrelay_path):
    if cmdline.startswith('-4', path_length) and  \
       (self.interface_name in cmdline):
        self.set_dhcp_pid(pid, version=4)
    elif cmdline.startswith('-6', path_length) and  \
          (self.interface_name in cmdline):
        self.set_dhcp_pid(pid, version=6)