OS-438, IPv6 support for Infoblox

* In https://jira.inca.infoblox.com:8443/browse/OPENSTACK-438 you may grab shell script to test functionality.
* Currently we support only DUID-LL stateful IPv6 (https://tools.ietf.org/html/rfc3315#section-9.4), so to get IPv6 in stateful mode run dhclient -6 -D LL eth0

Lines 174-179
```
        if self.dhcp_active or self.dhcp6_active or self.dns_active:
            self.restart()
        elif self._enable_dns_dhcp():
            self.interface_name = interface_name
            self.spawn_process()
```
We call self.dhcp_active before spawning processes. In this situation there is no PID.