New nova API call to mark nova-compute down

New API call is needed to change the state of compute service "down"
immediately. This allows usage of evacuate API without a delay. Also as
external system will make sure host is down and no VMs left running,
there will be no possibility to break shared storage or use same IPs
again.

Change-Id: I0a9f805df865aadd8a250ee13cbabdf44de9567d
JIRA: DOCTOR-5
Signed-off-by: Carlos Goncalves <carlos.goncalves@neclab.eu>