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
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index ac18490..80c60d5 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -210,6 +210,12 @@
 # Number of seconds to time out on waiting for a volume
 # to be available or reach an expected status
 build_timeout = 300
+# Runs Cinder multi-backend tests (requires 2 backend declared in cinder.conf)
+# They must have different volume_backend_name (backend1_name and backend2_name
+# have to be different)
+multi_backend_enabled = false
+backend1_name = LVM_iSCSI
+backend2_name = LVM_iSCSI_1
 
 [object-storage]
 # This section contains configuration options used when executing tests