Adds Cinder Multi-Backend Test

Adds a new file containing 1 test:
- test if the multi backend feature is working: "os-vol-attr:host"
  should return something like: "host@backend_name"
- test if the creation of a volume with a specific volume type is
  created in the good backend

2 backend (configured in cinder.conf) are required to work properly.
This patch introduces new tempest configuration variables:
- multi_backend_enabled: skip the tests if it's set to false
- backend1_name: name of the first backend
- backend2_name: name of a second backend
(backend1_name and backend2_name must be different)

Change-Id: I82d533e0737ffde54b3829f2a8b91c75641f2639
Merge "Adds Cinder Multi-Backend Test"