fixed syntax error relation-set, added missing backend config option

src/config.yaml

@@ -9,3 +9,7 @@ options:
     type: string
     default: iscsi
     description: "SAN protocol to use. Choose between iscsi or fc."
+  volume-backend-name:
+    type: string
+    description: "Service name to present to Cinder"
+    default:

src/lib/charm/openstack/cinder_purestorage.py

@@ -30,7 +30,7 @@ class PureStorageCharm(OpenStackCharm):
         else:
             relation_set(
                 relation_id=rel_id[0],
-                backend_name=config()['volume-backend-name'] or service_name(),
+                backend_name=config('volume-backend-name') or service_name(),
                 subordinate_configuration=json.dumps(
                     PureStorageSubordinateContext()()),
                 stateless=True,