Integrate the vpdu into the gateway

This takes the vpdu and tosses it into the gateway container, allowing any gateway (in vivian or on bare metal) to use a virtual PDU for starting DUT VMs for testing.

In order to use it, the existing vivian-dut make target has been taken over for use when the gateway is in vivian:

$ make vivian-dut OUTLET=3

When the gateway is on bare metal, use the discover endpoint API, then you can power on DUTs manually after they are added by the discover process (this also works with vivian):

$ vpdu-client --outlet 3 --on

When the DUT VM is started by the vpdu, a new window is added to the tmux dashboard. Currently the VM output is just tail'd (literally, using tail), so it's not possible to interact with it directly. This could possibly be improved in the future, unless someone knows how to do that easily now 😄

eye candy: