OPTE virtual gateway MAC address should be provided by Nexus

[bnaecker]

OPTE acts as a virtual gateway to guests, and so needs both an IP and MAC address. The IP is always reserved to be the first in the VPC subnet. The MAC address however, is currently hardcoded in the sled agent. The fact that it's hardcoded is OK, but that should be done in Nexus. The value really should be stored in Cockroach, so that it can be consistently controlled and we can avoid duplicates where applicable. Nexus will then need to supply the value to the sled agent, for example during instance provision requests.