Set reasonable defaults for new network values

Partially-implements blueprint add-mtu-and-dhcp-server-to-networks

Change-Id: Ic9c60071480abc5fced04dab5fc79ea44db1e10d
Determine shared ip from table instead of flag

Partially-implements blueprint add-mtu-and-dhcp-server-to-networks

Change-Id: Id7c6620704fef504d23cb42c5218251fa9e2dba7
Use mtu setting from table instead of flag

Partially-implements blueprint add-mtu-and-dhcp-server-to-networks

Change-Id: Ia401eb95ce8c81d104e59b0348e3474655b9a992
Allow dhcp_server to be set from new field

Now that dhcp_server and gateway can be different, update dnsmasq
to hand out the gateway if we have a single ip address and they
don't match.

Partially-implements blueprint add-mtu-and-dhcp-server-to-networks

Change-Id: Id040ce1fb0f889418815568ff6c80402b69bf09e