Update cinder-volume to enable multi volume support

* Added the service wrapper for multi binscripts
* Added a service_name to manager
* Added the configuration wrapper so the impls do not need
  to know if they are accessing config values from a option
  group or from the DEFAULT group
* Updated drivers to use the new configuration wrapper
* Added an example config set for multi backend
* Modified service to comply w/ common
* Added get_volume_stats to iscsi
* Added config for volume_backend_name

implements blueprint multi-volume-backends

Change-Id: Idb86ac5f1c06b9cd5061bef6a24f17727e9a58c1
Merge "Update cinder-volume to enable multi volume support"