perfdhcp should be able to generate traffic from multiple subnets

Right now perfdhcp is limited to a single subnet. The goal of this ticket is to:

• come up with a strategy to simulate traffic from multiple subnets
• extend perfdhcp code.

MR !691 (closed) resolves this issue by creating a testing hook which randomly select one of the available subnets and sends the response directly to the relay (perfdhcp) - works for v4 and v6 - the MR also makes #1132 (closed) pointless