Make host management more flexible

Some sites have their own /etc/hosts management
outside of puppet. This patch allows the data model
to drive hosts entries using the standard host
resource, instead of templating. This should only be used
when the fqdn is already provided in the hosts file.
Add dhcp firewall rules for linuxbridge

Linuxbridge requires forwarding between network
interfaces and the dhcp agent's veth pair to
function. This patch adds rudimentary support
for it.