Add new fields to the networks table

Adds a migration to add mtu, dhcp_server, enable_dhcp, 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 better-support-for-multiple-networks

Change-Id: I8278fb125e6e478b519c875eca9ebbf778d88d74
Merge "Add new fields to the networks table"