Adds host power actions support for Hyper-V

This patch introduces host power off and host reboot
support for the Nova Hyper-V driver.

Startup cannot be supported as the driver runs on the
host itself.

Implements: blueprint hyper-v-host-power-actions

Change-Id: I86ca35ff2527f643d72fb949870da69ce63277ae
Merge "Adds host power actions support for Hyper-V"