Clear shared-db.setup when departing shared-db

When shared-db is rejoined the charm will reconfigure the DB IFF DB
related flags are cleared.

Change-Id: Ieb575d6aa220f10c519425f1e1a6f59e29c512f1
Closes-Bug: #1887265

src/reactive/designate_handlers.py

@@ -242,3 +242,15 @@ def local_pools_updated():
         "Pools updated locally, restarting pool manager",
         level=hookenv.DEBUG)
     host.service_restart('designate-pool-manager')
+
+
+@reactive.when('shared-db.setup')
+@reactive.when_not('shared-db.connected')
+def reset_shared_db():
+    """Clear flags on shared-db departed.
+
+    When shared-db is rejoined the charm will reconfigure the DB IFF these
+    flags have been cleared. See LP Bug#1887265
+    """
+    reactive.remove_state('shared-db.setup')
+    reactive.remove_state('db.synched')

unit_tests/test_designate_handlers.py

@@ -39,6 +39,7 @@ class TestRegisteredHooks(test_utils.TestRegisteredHooks):
                 'expose_endpoint': ('dnsaas.connected', ),
                 'remote_pools_updated': (
                     'leadership.changed.pool-yaml-hash', ),
+                'reset_shared_db': ('shared-db.setup', ),
             },
             'when_not': {
                 'setup_amqp_req': ('amqp.requested-access', ),
@@ -50,6 +51,7 @@ class TestRegisteredHooks(test_utils.TestRegisteredHooks):
                 'create_servers_and_domains': ('domains.created', ),
                 'run_assess_status_on_every_hook': (
                     'dont-set-assess-status', ),
+                'reset_shared_db': ('shared-db.connected', ),
             },
             'when_any': {
                 'set_dns_config_available': (