Specify DHCP servers on a per-VLAN basis to reflect new schema

- DHCP servers are now specified on a per-VLAN basis, so the device data should be updated to reflect this. VLANs and their properties (including DHCP servers) should be specified in the various T0 topology files and used in the generation of minigraphs.
- The DHCP relay test should also be modified to support multiple VLANs, ultimately running a test on each configured VLAN.