Support forcing service down

Registered by dongwenjuan

Add a new API to force service (cinder-volume) down in Cinder. Here is the
blueprint: https://blueprints.launchpad.net/cinder/+spec/mark-services-down.
Cinder client should support the same. Therefore API needs to be added:

usage: cinder service-force-down <hostname> <binary> [--unset]

Force service state down.

Positional arguments:
   <hostname> Name of host.
   <binary> Service binary.

Optional arguments:
    without optional: Set the force down of service (default:True)
    --unset: Unset the force down of service(False).

Blueprint information

Status:
Not started
Approver:
Sean McGinnis
Priority:
Undefined
Drafter:
dongwenjuan
Direction:
Needs approval
Assignee:
dongwenjuan
Definition:
New
Series goal:
None
Implementation:
Unknown
Milestone target:
None

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