Add volume_backend_name to render in cinder.conf

Change-Id: I7e8f0c19047a20a178f6d16017f3c424c856c918

config.yaml

@@ -20,10 +20,12 @@ options:
   volume-backend-name:
     type: string
     description: |
-        A volume back-end name which is specified in the cinder.conf file.
-        This is also used while assigning a back-end name to the Nimble
-        volume type.
-    default: cinder_nimblestorage
+        Volume backend name for the backend. The default value is the
+        application name in the Juju model, e.g. "cinder-nimble-mybackend"
+        if it's deployed as `juju deploy cinder-nimble cinder-nimble-mybackend`
+        A common backend name can be set to multiple backends with the
+        same characters so that those can be treated as a single virtual
+        backend associated with a single volume type.
   use-multipath-for-image-xfer:
     type: boolean
     description: |

src/charm.py

@@ -46,9 +46,12 @@ class CinderNimblestorageCharm(CinderStoragePluginCharm):
         cget = charm_config.get
 
         volume_driver = cget('volume-driver')
+        volume_backend_name = cget(
+            'volume-backend-name') or self.framework.model.app.name
 
         raw_options = [
             ('volume_driver', VOLUME_DRIVERS[volume_driver]),
+            ('volume_backend_name', volume_backend_name),
             ('san_ip', cget('san-ip')),
             ('san_login', cget('san-login')),
             ('san_password', cget('san-password')),