Allow DHCP on isolated sites without requiring a gateway or DNS forwarders

[ArshSSandhu]

Summary

Manager currently requires a site to have both a gateway and DNS forwarders before the agent applies the dnsmasq configuration.

This prevents DHCP from working in a fully isolated or air-gapped environment where a gateway or upstream DNS server may intentionally not exist.

Steps to Reproduce

1. Create a site in Manager with:
   • Internal domain
   • DHCP range
   • Subnet mask
   • No gateway
   • No DNS forwarders
2. Allow the Manager agent to apply the site configuration.
3. Start a container configured to receive its IPv4 address through DHCP.
4. Check the container network configuration and dnsmasq listeners.

Actual Behavior

The dnsmasq configuration is skipped when gateway or DNS forwarders are missing.

As a result, DHCP does not listen on UDP port 67 and the container does not receive an IPv4 address.

Expected Behavior

DHCP and internal DNS should be able to operate on an isolated network without requiring an external gateway or upstream DNS forwarders.

Proposed Direction

Allow dnsmasq DHCP configuration when the internal domain, DHCP range, and subnet mask are configured, while treating gateway and DNS forwarders as optional for isolated sites.

[ArshSSandhu]

@runleveldev