add volume billing events support in nova

Registered by Yun Shen

Generate billing events for the following Volume Actions:
- Create Volume
- Attach Voulme
- Snapshot Volume
- Detattch Volume
- Delete Volume

Event generation should follow the pattern used for VM events

The Billing process will recieve these events and generate a billing record for each one.

A volume-usage-audit script (peer of the instance-usage-audit scrpit) wil generate periodic billing entries

Blueprint information

Status:
Complete
Approver:
None
Priority:
Undefined
Drafter:
None
Direction:
Needs approval
Assignee:
None
Definition:
Obsolete
Series goal:
None
Implementation:
Unknown
Milestone target:
None
Completed by
Vish Ishaya

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This should be moved to cinder

(?)

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