Sheepdog: Fix a problem about multi backend

Sheepdog: Fix a problem about multi backend

Current Sheepdog driver does not support Cinder multi backend. It can
connect at most one Sheepdog cluster defined in the [DEFAULT] section
in the cinder.conf.

This patch makes Sheepdog driver support Cinder multi backend.
I think this should be treated as bug fix rather than enhancement
(i.e. adding a new feature) because Cinder itself has been supported
multi backend from Grizzly.

You can use multi Sheepdog clusters (for example, one of them has
low-performance SATA disk and another has high-performance SSD) with
cinder.conf like as below:

  [DEFAULT]
  enabled_backends=sheep0,sheep1
  [sheep0]
  volume_driver=cinder.volume.drivers.sheepdog.SheepdogDriver
  sheepdog_store_address=127.0.0.1
  sheepdog_store_port=7000
  volume_backend_name=sheep_SATA
  [sheep1]
  volume_driver=cinder.volume.drivers.sheepdog.SheepdogDriver
  sheepdog_store_address=127.0.0.1
  sheepdog_store_port=7001
  volume_backend_name=sheep_SSD

DocImpact

Closes-Bug: #1514353

Change-Id: I484430e52aa0661bf1cddf9939ad308e274317e2