Add storage-backend subordinate relation

Implement initial support for container-scoped storage-backend
relation to integrate nova-compute with storage providers.

Transition to blocked state if multiple storage backend
relations are connected

Change-Id: I03e67731df0263887d2d0f671750f420d6e62c1c

hooks/nova_compute_utils.py

@@ -1000,6 +1000,9 @@ def check_optional_config_and_relations(configs):
         except ValueError as e:
             return ('blocked', 'Invalid configuration: {}'.format(str(e)))
 
+    if len(relation_ids('storage-backend')) > 1:
+        return 'blocked', "Multiple storage backends are not supported"
+
     # return 'unknown' as the lowest priority to not clobber an existing
     # status.
     return "unknown", ""

metadata.yaml

@@ -54,6 +54,9 @@ requires:
     interface: vault-kv
   ironic-api:
     interface: baremetal
+  storage-backend:
+    interface: storage-backend
+    scope: container
 peers:
   compute-peer:
     interface: nova