Scaling backup service

Scaling backup service

Currently the cinder backup service is tightly coupled to the cinder
volume service in ways that prevent scaling out backup services
horizontally across multiple physical nodes.

This patch is to loosen this coupling to enable backup processes
to run on multiple nodes without having to be colocated with volume
services.

The following works are not included in this patch:
1. Remote attach snapshot.
2. Vendor specific work.
3. Remove current backup_volume in driver.
4. Rolling upgrades.

DocImpact
Change-Id: I743e676372703e74178c79683dd622d530981e04
Partial-Implements: bp scalable-backup-service
Co-Authored-By: Tom Barron <tpb@dyncloud.net>