Support HA DHCP with "leader"

[BeryJu]

Currently all gravity instances that have the DHCP role configured will listen and reply to DHCP requests. This is not ideal when using multiple gravity instances in a single L2 network, where ideally a single instance should be a "leader" using the etcd keep-alive feature, and then another node can take over if needed.

The somewhat difficult part is figuring out which nodes are in the same L2 network, not sure if we can rely on trying to find the overlap based on INSTANCE_IP, the local interface and netmask and seeing what other nodes have, made especially more difficult since we're running in a container.