Adds default route to DHCP namespace for upstream name resolution.

Any time the DHCP server is updated this code will maintain a default
route in the DHCP namespace using the gateway_ip attribute of the
first DHCP-enabled IPV4 subnet in the list of subnets where gateway_ip
is not None.  This strategy uses the same gateway ip that the DHCP
server hands to the VMs on the network.

Fixes: Bug #1181378

Conflicts:

	quantum/agent/dhcp_agent.py

Change-Id: I0807550a848e1b610c7775d215643ad9c83629ed
(cherry picked from commit 5dd0cf15463e181b92c8809c2dcc7603bb156a0e)