Enable VM DHCP request to reach DHCP agent

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.

New rules will be added that enable DHCP requests to pass.

This fixes bug 1131223

Change-Id: I5117458d6a4c0fe2e333f6f3c2f902672e6293e5
Merge "Enable VM DHCP request to reach DHCP agent" into stable/folsom