Fix DHCP status for DHCP relay (#1173)

[vtapia]

Done

• Set DHCP status to "Relayed via ##" instead of "No DHCP" when DHCP is relayed, both in the node and the network pages

QA

In a machine with two fabrics (e.g. fabric-0 and fabric-1) with a different subnet each:

• Enable DHCP in fabric-0's vlan page
• Enable DHCP relay in fabric-1's vlan page
• /MAAS/#/networks?by=fabric should show "Relayed via fabric-0.untagged" for fabric-1 instead of "No DHCP"
• After deploying a machine with an interface on fabric-1, the machine should show "Relayed via fabric-0.untagged" instead of "No DHCP" in /MAAS/#/machine/$id

Fixes

Fixes #1173 .

Launchpad Issue

lp#1879979

[Caleb-Ellis]

This is great! Thanks for that @vtapia - I'll add a follow-up branch for the network tab in machine details, add tests and play around with the table widths so it's not so heavily truncated.