Add mtu, dhcp_server, share_address to networks

Adds a migration to add mtu, dhcp_server, and share_address to the
networks table. The field for dhcp_server defaults to gateway, since
gateway is currently used to determine the dhcp_server value. The
value for mtu and share address is determined by a flag, so copy the
value from the flag into the table during the migration.

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

Change-Id: I8278fb125e6e478b519c875eca9ebbf778d88d74
Adds mtu, dhcp_server, share_address to object

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

Change-Id: I318791a9d7f53534b3383221cdfb7dfe63dd116b
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.

Change-Id: Id040ce1fb0f889418815568ff6c80402b69bf09e