When using the firewall driver IptablesFirewallDriver and the
default INPUT and FORWARD rules are DISCARD then the DHCP
request from the VM is discarded prior to getting to the dnsmasq.
A new rule will be added that enables DHCP requests to pass.
This fixes bug 1131223
Change-Id: I50fad5b63c3c4b22a5d828e3e89353c1ed723332