Do not fan out DB sync to all cells

Support syncing cell conductor DBs on a per cell basis so that a
cell can be upgraded in isolation.

Change-Id: I357510a05cacbb9f2cdddbbe9fd06acbce9468ac

docker/nova/nova-conductor/extend_start.sh

@@ -1,13 +1,13 @@
 #!/bin/bash
 
 if [[ "${!KOLLA_BOOTSTRAP[@]}" ]]; then
-    nova-manage db sync
+    nova-manage db sync --local_cell
     nova-manage db online_data_migrations
     exit 0
 fi
 
 if [[ "${!KOLLA_UPGRADE[@]}" ]]; then
-    nova-manage db sync
+    nova-manage db sync --local_cell
     exit 0
 fi