Add backend-availability-zone config option

Add support for configuring a backend availability zone.

Closes-Bug: #2058407
Change-Id: I1c6620b72923260a52e288911649c2e0e5ec3c61
(cherry picked from commit 7df1d81f92)
(cherry picked from commit e5ea85ab62)

src/config.yaml

@@ -1,4 +1,10 @@
 options:
+  backend-availability-zone:
+    default:
+    type: string
+    description: |
+      Availability zone name of this volume backend. If set, it will
+      override the default availability zone.
   driver-source:
     type: string
     default:

src/lib/charm/openstack/cinder_purestorage.py

@@ -42,6 +42,11 @@ class CinderpurestorageCharm(
             ('volume_backend_name', service),
             ('allowed_direct_url_schemes', 'cinder')]
 
+        backend_az = self.config.get('backend-availability-zone')
+        if backend_az:
+            driver_options.append(
+                ('backend_availability_zone', backend_az))
+
         if self.config.get('protocol') == 'nvme-roce':
             if self.config.get('nvme-cidr'):
                 nvme_roce.extend([('pure_nvme_cidr',