Nexus needs to allocate Boundary Services IP address and VNI

[bnaecker]

We're currently hardcoding the IP address and VNI for Boundary Services in the sled agent. That's fine while Boundary Services is being built, but we really need to do that creation in Nexus and store the values in the database. They can still be hardcoded, but must be done in Nexus and communicated to the sled agent, for example during instance provision. They'll also need to be forward to Dendrite itself, so it can program the switch accordingly.