Add nexenta backend backup driver

Registered by Mikhail Khodos

Add 'nexenta' driver to support any type of cinder volume backup/restore to/from NexentaStor backend. The driver should support all types of volumes to be backed up to and restored from NexentaStor backend. This will be done naively as we don't know any specifics about volume to be backed up.
In case we perform NexentaStor -> NexentaStor backup(restore) , we will use ZFS-specific optimizations do perform the operation more efficiently.
Please visit https://review.openstack.org/#/c/47005/ for code review.

Blueprint information

Status:
Complete
Approver:
John Griffith
Priority:
Undefined
Drafter:
Mikhail Khodos
Direction:
Needs approval
Assignee:
Mikhail Khodos
Definition:
Obsolete
Series goal:
None
Implementation:
Unknown
Milestone target:
None
Completed by
Sean McGinnis

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